keyboard: Disconnect from input source manager when destroying indicator

keyboard: Disconnect from input source manager when destroying indicator

When an InputSourceIndicator is destroyed, the InputSourceManager it was
connected to could (and probably will) outlive it (since the manager is
a singleton). If the InputSourceManager emits any subsequent signals,
the callbacks from the finalised InputSourceIndicator could be invoked,
and will reference finalised objects.

This can be triggered by running `pkexec true` from a gnome-terminal
window, then calling `pkill pkexec` from another terminal (on a
different VT or via SSH). This causes the dialogue to be cancelled by
polkitd.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/357