[server] Support sub-surfaces from PointerInterface

Summary:
The idea behind this change is to make the existance of sub-surfaces
an implementation detail for pointer events. The user of the library
does not need to care about on which sub-surface the pointer is on.
It only needs to care about the main surface and passes the focus to
the main surface.

Internally the PointerInterface takes care of sending the enter to
the sub-surface at the current pointer position. Also whenever the
pointer position changes, the PointerInterface evaluates whether it
triggered a change for the focused sub-surface and sends enter/leave
events accordingly. If the focused sub-surface does not change, it
sends motion events as normally, but of course under consideration
of the sub-surface position.

Overall this means that from pointer usage perspective a user of the
library doesn't need to care about the fact that there are sub-surfaces
at all. The library does the correct thing for it.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1329