* 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
* Add `TextFormat::extra_letter_spacing`
* Add control of line height
* Add to text layout demo
* Move the text layout demo to its own window in the demo app
* Fix doclink
* Better document points vs pixels
* Better documentation and code cleanup
* use font metrics in layout
* properly center scaled fonts
* adjust docs
* fix raised text
* fix easymark viewer small text alignment
caused by variable row heights
* eframe web: Add WebInfo::user_agent
* Deprecate `Modifier::ALT_SHIFT`
* Add code for formatting Modifiers and Key
* Add type KeyboardShortcut
* Code cleanup
* Add Context::os/set_os to query/set what OS egui believes it is on
* Add Fonts::has_glyph(s)
* Add helper function for formatting keyboard shortcuts
* Faster code
* Add way to set a shortcut text on menu buttons
* Cleanup
* format_keyboard_shortcut -> format_shortcut
* Add TODO about supporting more keyboard sumbols
* Modifiers::plus
* Use the new keyboard shortcuts in emark editor demo
* Explain why ALT+SHIFT is a bad modifier combo
* Fix doctest
Emil Ernerfeldt
lictex_