[server] Properly initialize Cursor

Summary:
The Cursor wasn't properly initialized. E.g. the damage signal didn't
get connected resulting in the server not noticing when the cursor
changes. The damage only got connected if a new cursor got instelled by
the client on the same pointer.

This change ensures that the Cursor is properly initialized by calling
into the same method as when the cursor changed.

The tests are extended by a new test case for damaging the surface.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1022