feat: [Keyboard Layout applet] add spare layouts support

For now, X11 only

To support spare layouts in new Keyboard Layouts applet via existing
DBus API, they are appended to the main ones and passed to the applet
alongside in a row.
When spare layout selected in applet's context menu, layout swapping
logic takes place - the spare layout activates and placed in last
position of the main layouts loop, preempting previous layout there in
current keymap.
That way the actual keymap can be dynamically modified from the menu,
without reconfiguring in KCM.
No DBus API/applet changes are needed currently.

CCBUG: 430661