* 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`.
## TODO
* [x] Test
5d2e192927