Implement table row selection and hover highlighting (#3347)

* Based on #3105 by @vvv.

## Additions and Changes

- Add `TableBuilder::sense()` and `StripBuilder::sense()` to enable
detecting clicks or drags on table and strip cells.
- Add `TableRow::select()` which takes a boolean that sets the highlight
state for all cells added after a call to it. This allows highlighting
an entire row or specific cells.
- Add `TableRow::response()` which returns the union of the `Response`
of all cells added to the row up to that point. This makes it easy to
detect interactions with an entire row. See below for an alternative
design.
- Add `TableRow::index()` and `TableRow::col_index()` helpers.
- Remove explicit `row_index` from callback passed to
`TableBody::rows()` and `TableBody::heterogeneous_rows()`, possible due
to the above. This is a breaking change but makes the callback
compatible with `TableBody::row()`.
- Update Table example to demonstrate all of the above.

## Design Decisions

An alternative design to `TableRow::response()` would be to return the
row response from `TableBody`s `row()`, `rows()` and
`heterogeneous_rows()` functions. `row()` could just return the
response. `rows()` and `heterogeneous_rows()` could return a tuple of
the hovered row index and that rows response. I feel like this might be
the cleaner soluction if only returning the hovered rows response isn't
too limiting.

I didn't implement `TableBuilder::select_rows()` as described
[here](https://github.com/emilk/egui/pull/3105#issuecomment-1618062533)
because it requires an immutable borrow of the selection state for the
lifetime of the `TableBuilder`. This makes updating the selection state
from within the body unnecessarily complicated. Additionally the current
design allows for selecting specific cells, though that could be
possible by modifying `TableBuilder::select_rows()` to provide row and
column indices like below.

```rust
pub fn select_cells(is_selected: impl Fn(usize, usize) -> bool) -> Self
```

## Hover Highlighting

EDIT: Thanks to @samitbasu we now have hover highlighting too.

~This is not implemented yet. Ideally we'd have an api that allows to
choose between highlighting the hovered cell, column or row. Should
cells containing interactive widgets, be highlighted when hovering over
the widget or only when hovering over the cell itself? I'd like to
implement that before this gets merged though.~

Feedback is more than welcome. I'd be happy to make any changes
necessary to get this merged.

* Closes #1519
* Closes #1553
* Closes #3069

---------

Co-authored-by: Samit Basu <basu.samit@gmail.com>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_extras/src/strip.rs

@@ -1,5 +1,5 @@
 use crate::{
-    layout::{CellDirection, CellSize, StripLayout},
+    layout::{CellDirection, CellSize, StripLayout, StripLayoutFlags},
     sizing::Sizing,
     Size,
 };
@@ -46,6 +46,7 @@ pub struct StripBuilder<'a> {
     sizing: Sizing,
     clip: bool,
     cell_layout: egui::Layout,
+    sense: egui::Sense,
 }
 
 impl<'a> StripBuilder<'a> {
@@ -55,8 +56,9 @@ impl<'a> StripBuilder<'a> {
         Self {
             ui,
             sizing: Default::default(),
-            cell_layout,
             clip: false,
+            cell_layout,
+            sense: egui::Sense::hover(),
         }
     }
 
@@ -74,6 +76,13 @@ impl<'a> StripBuilder<'a> {
         self
     }
 
+    /// What should strip cells sense for? Default: [`egui::Sense::hover()`].
+    #[inline]
+    pub fn sense(mut self, sense: egui::Sense) -> Self {
+        self.sense = sense;
+        self
+    }
+
     /// Allocate space for one column/row.
     #[inline]
     pub fn size(mut self, size: Size) -> Self {
@@ -102,7 +111,12 @@ impl<'a> StripBuilder<'a> {
             self.ui.available_rect_before_wrap().width(),
             self.ui.spacing().item_spacing.x,
         );
-        let mut layout = StripLayout::new(self.ui, CellDirection::Horizontal, self.cell_layout);
+        let mut layout = StripLayout::new(
+            self.ui,
+            CellDirection::Horizontal,
+            self.cell_layout,
+            self.sense,
+        );
         strip(Strip {
             layout: &mut layout,
             direction: CellDirection::Horizontal,
@@ -125,7 +139,12 @@ impl<'a> StripBuilder<'a> {
             self.ui.available_rect_before_wrap().height(),
             self.ui.spacing().item_spacing.y,
         );
-        let mut layout = StripLayout::new(self.ui, CellDirection::Vertical, self.cell_layout);
+        let mut layout = StripLayout::new(
+            self.ui,
+            CellDirection::Vertical,
+            self.cell_layout,
+            self.sense,
+        );
         strip(Strip {
             layout: &mut layout,
             direction: CellDirection::Vertical,
@@ -171,9 +190,11 @@ impl<'a, 'b> Strip<'a, 'b> {
     #[cfg_attr(debug_assertions, track_caller)]
     pub fn cell(&mut self, add_contents: impl FnOnce(&mut Ui)) {
         let (width, height) = self.next_cell_size();
-        let striped = false;
-        self.layout
-            .add(self.clip, striped, width, height, add_contents);
+        let flags = StripLayoutFlags {
+            clip: self.clip,
+            ..Default::default()
+        };
+        self.layout.add(flags, width, height, add_contents);
     }
 
     /// Add an empty cell.