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
     }

crates/epaint/src/text/text_layout_types.rs

@@ -426,6 +426,9 @@ impl TextWrapping {
 ///   from `egui::InputState` and can change at any time.
 /// - The atlas has become full. This can happen any time a new glyph is added
 ///   to the atlas, which in turn can happen any time new text is laid out.
+///
+/// The name comes from typography, where a "galley" is a metal tray
+/// containing a column of set type, usually the size of a page of text.
 #[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct Galley {
@@ -672,6 +675,11 @@ impl Galley {
         }
     }
 
+    /// Returns a 0-width Rect.
+    pub fn pos_from_cursor(&self, cursor: &Cursor) -> Rect {
+        self.pos_from_pcursor(cursor.pcursor) // pcursor is what TextEdit stores
+    }
+
     /// Returns a 0-width Rect.
     pub fn pos_from_pcursor(&self, pcursor: PCursor) -> Rect {
         let mut it = PCursor::default();
@@ -708,8 +716,13 @@ impl Galley {
     }
 
     /// Returns a 0-width Rect.
-    pub fn pos_from_cursor(&self, cursor: &Cursor) -> Rect {
-        self.pos_from_pcursor(cursor.pcursor) // pcursor is what TextEdit stores
+    pub fn pos_from_ccursor(&self, ccursor: CCursor) -> Rect {
+        self.pos_from_cursor(&self.from_ccursor(ccursor))
+    }
+
+    /// Returns a 0-width Rect.
+    pub fn pos_from_rcursor(&self, rcursor: RCursor) -> Rect {
+        self.pos_from_cursor(&self.from_rcursor(rcursor))
     }
 
     /// Cursor at the given position within the galley.