backends: Add a keymap-layout-group-changed signal

We'll need this in the wayland frontend to send a modifiers event to
clients.

Note that on X11 this isn't needed because key events include the
group index encoded in modifier state. If we ever want to make the
wayland frontend work with the X11 backend we'll handle it then.

https://bugzilla.gnome.org/show_bug.cgi?id=736433