This defers the destruction of wlr_keyboard_groups until idle. This is
to prevent the keyboard group's keyboard from being destroyed in the
middle of handling a keyboard event. This would occur when changing the
keymap of the last keyboard in a group with a keyboard binding. The
prevents crashing when attempting to update the xkb state of the
keyboard group's keyboard. The sway_keyboard_group is still immediately
destroyed so that the group is no longer used