These changes will happen in the input event management code, so let them
be emitted via the MetaSeatImpl, as that's what we'll have neat access to.
The ClutterSeat signals are now emitted from there.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>
Move most of the functional bits (those meant to run on a standalone
thread) to a MetaSeatImpl object. This object is managed by the MetaSeatImpl
and not exposed outside the friend MetaSeatNative/MetaInputDeviceNative/
MetaInputSettings classes.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>