* Closes https://github.com/emilk/egui/issues/3816
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
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Closes <https://github.com/emilk/egui/issues/3474>.
* Allow widget focus change with keyboard arrows
* remove set id function
* docs
* Emilk feedback round 1
* Fix compile error
* undo example
* Move out functions from range to memory.rs
* remove contains range
* Use docstrings
* code cleanup
* Improve candidate logic
* More tweaks
* Less `pub`
---------
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
