input: Add basic seat handling

This allows for basic input handling, to better customize how we handle
out seat capabilities to the client. One useful outcome of this is the
fact that we re-advertise to the client that in some situations there
might not be a pointer available. That should let the client know as to
avoid creating a pointer surface.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I4624a956264e9fa12a86da005944e9b426dde10a