Get the Logind seat for our logind session
In order to switch virtual terminals through logind we need to know
the Seat object for our session.

Switch virtual teminal through logind seat

When we get the key code for virtual terminal switching we pass it
to Logind instead of going through the "legacy" ioctl interface.

Integrate libinput only through Logind instead of relying on VirtualTerminal

All the important information like whether we are on the active vt are
also provided by Logind. So instead of integrating with VirtualTerminal,
which integrates with Logind, we can just integrate with Logind directly.

[plugins/drm] Drop dependency on VirtualTerminal

Only integrates with Logind to find out when the session is active.

Fixup with input

[platforms/hwcomposer] Remove integration with VirtualTerminal

The hwcomposer plugin doesn't need it. It was only created for the
need of libinput relying on it. As libinput doesn't need it any more,
hwcomposer doesn't need to integrate with VirtualTerminal.