Selectable text in Labels (#3814)

* Closes https://github.com/emilk/egui/issues/3804

Add ability to select the text in labels with mouse-drag, double-click,
and keyboard (once clicked).
Hit Cmd+C to copy the text. If everything of a label with elided text is
selected, the copy command will copy the full non-elided text. IME and
accesskit _should_ work, but is untested.

You can control wether or not text in labels is selected globally in
`style.interaction.selectable_labels` or on a per-label basis in
`Label::selectable`. The default is ON.

This also cleans up the `TextEdit` code somewhat, fixing a couple
smaller bugs along the way.

This does _not_ implement selecting text across multiple widgets. Text
selection is only supported within a single `Label`, `TextEdit`, `Link`
or `Hyperlink`.


![label-text-selection](https://github.com/emilk/egui/assets/1148717/c161e819-50da-4b97-9686-042e6abf3564)


## TODO
* [x] Test

crates/epaint/src/text/text_layout_types.rs

@@ -142,7 +142,7 @@ impl LayoutJob {
         }
     }
 
-    #[inline(always)]
+    #[inline]
     pub fn is_empty(&self) -> bool {
         self.sections.is_empty()
     }
@@ -618,22 +618,45 @@ impl Row {
 }
 
 impl Galley {
-    #[inline(always)]
+    #[inline]
     pub fn is_empty(&self) -> bool {
         self.job.is_empty()
     }
 
     /// The full, non-elided text of the input job.
-    #[inline(always)]
+    #[inline]
     pub fn text(&self) -> &str {
         &self.job.text
     }
 
+    #[inline]
     pub fn size(&self) -> Vec2 {
         self.rect.size()
     }
 }
 
+impl AsRef<str> for Galley {
+    #[inline]
+    fn as_ref(&self) -> &str {
+        self.text()
+    }
+}
+
+impl std::borrow::Borrow<str> for Galley {
+    #[inline]
+    fn borrow(&self) -> &str {
+        self.text()
+    }
+}
+
+impl std::ops::Deref for Galley {
+    type Target = str;
+    #[inline]
+    fn deref(&self) -> &str {
+        self.text()
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 /// ## Physical positions