desktop-shell: Keep track of seats

These two patches avoid an illegal access happening at compositor exit, due to the fact that weston_desktop is no longer available (introduced by 719ca873) and focus_out events are still emitted.

This is combined with the fact that we don't have a separation between xdg-shell activation and keyboard focus (which should be addressed by !570 (merged)).

The first patch just adds support for keeping track of the shell_seat while the latter removes the missing listeners (keyboard and caps changed).

!570 (merged) would make this illegal access not possible because the keyboard focus handler is removed entirely, but even so, let's keep it consistent with kiosk-shell, which already does this.