keybindings: Freeze and ungrab the keyboard only on X11 backend

This isn't needed in the native backend because we control all the
input event flow there.

https://bugzilla.gnome.org/show_bug.cgi?id=736433
wayland-keyboard: Handle keymap-changed signal

We need to inform wayland clients of new keymaps.

https://bugzilla.gnome.org/show_bug.cgi?id=736433
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
wayland-keyboard: Handle keymap-layout-group-changed signal

We need to send a modifiers event to wayland clients when the layout
group changes.

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