Add support for the `Cut`, `Copy`, `Paste`,
`Undo`, and `Redo` keys to text views, and
`Find` to the file chooser widget.

These keys can be found on old keyboards,
but they will also be emitted by
custom layouts like [Extend](https://dreymar.colemak.org/layers-extend.html

).

Signed-off-by: Jules Bertholet <julesbertholet@quoi.xyz>