Cross-widget text select (#3870)

* Closes https://github.com/emilk/egui/issues/3816


![cross-widget-text-selection](https://github.com/emilk/egui/assets/1148717/5582b9d2-8b04-47b7-9637-f48e44064a70)

Turn off with `style.interaction.multi_widget_text_select`.

There is an API for this in `LabelSelectionState`, but it's pretty
bare-bones.

This became really hairy implementation-wise, but it works decently
well.

# Limitations
* Drag-select to scroll doesn't work
* A selection disappears if you scroll past one of its end-points
* Only the text of labels and links are selectable
 
## TODO
* [x] An option to turn it off
* [x] An API for querying about the selected text, and to deselect it.
* [x] Scrolling past selection behaves weird
* [x] Shift-click to select a range

crates/epaint/src/text/cursor.rs

@@ -17,6 +17,7 @@ pub struct CCursor {
 }
 
 impl CCursor {
+    #[inline]
     pub fn new(index: usize) -> Self {
         Self {
             index,
@@ -25,9 +26,17 @@ impl CCursor {
     }
 }
 
+impl From<Cursor> for CCursor {
+    #[inline]
+    fn from(c: Cursor) -> Self {
+        c.ccursor
+    }
+}
+
 /// Two `CCursor`s are considered equal if they refer to the same character boundary,
 /// even if one prefers the start of the next row.
 impl PartialEq for CCursor {
+    #[inline]
     fn eq(&self, other: &Self) -> bool {
         self.index == other.index
     }
@@ -106,6 +115,7 @@ pub struct PCursor {
 /// Two `PCursor`s are considered equal if they refer to the same character boundary,
 /// even if one prefers the start of the next row.
 impl PartialEq for PCursor {
+    #[inline]
     fn eq(&self, other: &Self) -> bool {
         self.paragraph == other.paragraph && self.offset == other.offset
     }