Summary:
There are two types of constraints supported:
1. Pointer confinement
2. Pointer locking

In the case of confinement the pointer is confined to a given region of
the surface. This is comparable to general operation where the pointer
is confined to the screen region.

In the second case the pointer gets locked. That means it cannot move at
all. No further position updates are provided, only relative motion
events can go to the application. There is a hint about cursor position
update on unlock which is not yet implemented in KWayland::Server, thus
also not in this change.

The implementation in KWin grants the requests for pointer constraints
when the pointer enters the constrained region, either by pointer
movement or by e.g. stacking order changes. There is no confirmation
from user required to enter that mode. But we want to show an OSD when
the pointer gets constrained, this is not yet implemented, though.

Breaking an active constraint is relatively ea...