Previously only the list of allowed keymaps could be modified and
applied to the system.
Add a new button to activate the selected keymap from the list. When
applying the changes to the system, also apply the active keymap.
In the Keymap Settings dialog, a check was missing when the Keymap
selection dialog was cancelled.
Not checking the return value causes an empty string to be added to
the keymap list.
I've attempted to handle the errors gracefully where it was clear how to
do so, and simple, but a lot of this was just adding
`release_value_but_fixme_should_propagate_errors()` in places.
Right now, this is a bit of a hack. We can't set a keymap to only apply
to the test area, so we set the system keymap instead, while also
keeping track of the "real" current keymap. Whenever the settings are
applied, we update what that "real" keymap is, and when we exit, we
revert to that keymap.
Basically, it behaves as you would expect, apart from it also affecting
other applications you are typing in while the KeyboardSettings window
is open with a different keymap selected.
