egui_extras::Table improvements (#2369)

* Use simple `ui.interact` for the resize line

* Introduce TableReizeState

* Simplify some code

* Add striped options to table demo

* Auto-size table columns by double-clicking the resize line

* Table: add option to auto-size the columns

* Table: don't let column width gets too small, unless clipping is on

* egui_extras: always use serde

Otherwise using `get_persisted` etc is impossible,
and working around that tedious.

* Avoid clipping last column in a resizable table

* Some better naming

* Table: Use new `Column` for setting column sizes and properties

Also make `clip` a per-column property

* All Table:s store state for auto-sizing purposes

* Customize each column wether or not it is resizable

* fix some auto-sizing bugs

* Fix shrinkage of adaptive column content

* Rename `scroll` to `vscroll` for clarity

* Add Table::scroll_to_row

* scroll_to_row takes alignment

* Fix bug in table sizing

* Strip: turn clipping OFF by default, because it is dangerous and sucks

* Add TableBody::mac_rect helper

* Table: add options to control the scroll area height.

* Docstring fixes

* Cleanup

crates/egui/src/containers/scroll_area.rs

@@ -66,6 +66,10 @@ pub struct ScrollAreaOutput<R> {
     /// The current state of the scroll area.
     pub state: State,
 
+    /// The size of the content. If this is larger than [`Self::inner_rect`],
+    /// then there was need for scrolling.
+    pub content_size: Vec2,
+
     /// Where on the screen the content is (excludes scroll bars).
     pub inner_rect: Rect,
 }
@@ -198,6 +202,8 @@ impl ScrollArea {
 
     /// Set the horizontal and vertical scroll offset position.
     ///
+    /// Positive offset means scrolling down/right.
+    ///
     /// See also: [`Self::vertical_scroll_offset`], [`Self::horizontal_scroll_offset`],
     /// [`Ui::scroll_to_cursor`](crate::ui::Ui::scroll_to_cursor) and
     /// [`Response::scroll_to_me`](crate::Response::scroll_to_me)
@@ -209,6 +215,8 @@ impl ScrollArea {
 
     /// Set the vertical scroll offset position.
     ///
+    /// Positive offset means scrolling down.
+    ///
     /// See also: [`Self::scroll_offset`], [`Ui::scroll_to_cursor`](crate::ui::Ui::scroll_to_cursor) and
     /// [`Response::scroll_to_me`](crate::Response::scroll_to_me)
     pub fn vertical_scroll_offset(mut self, offset: f32) -> Self {
@@ -218,6 +226,8 @@ impl ScrollArea {
 
     /// Set the horizontal scroll offset position.
     ///
+    /// Positive offset means scrolling right.
+    ///
     /// See also: [`Self::scroll_offset`], [`Ui::scroll_to_cursor`](crate::ui::Ui::scroll_to_cursor) and
     /// [`Response::scroll_to_me`](crate::Response::scroll_to_me)
     pub fn horizontal_scroll_offset(mut self, offset: f32) -> Self {
@@ -541,18 +551,20 @@ impl ScrollArea {
         let id = prepared.id;
         let inner_rect = prepared.inner_rect;
         let inner = add_contents(&mut prepared.content_ui, prepared.viewport);
-        let state = prepared.end(ui);
+        let (content_size, state) = prepared.end(ui);
         ScrollAreaOutput {
             inner,
             id,
             state,
+            content_size,
             inner_rect,
         }
     }
 }
 
 impl Prepared {
-    fn end(self, ui: &mut Ui) -> State {
+    /// Returns content size and state
+    fn end(self, ui: &mut Ui) -> (Vec2, State) {
         let Prepared {
             id,
             mut state,
@@ -847,7 +859,7 @@ impl Prepared {
 
         state.store(ui.ctx(), id);
 
-        state
+        (content_size, state)
     }
 }