Send output enter/leave events to surfaces
A surface has an API to know which screen(s) it's on, this is useful
especially for knowing the scale it should render at.
In practice Qt currently doesn't do anything with this information; but
that's set to change.
Test_helpers is changed as we need to create output objects in order for
wl_surface to map them.
Closes task T4467
Attached unit test.
As mentioned above, it doesn't have any real world impact currently, so not a lot
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: johanhelsing, graesslin, plasma-devel, kwin, KWin
Maniphest Tasks: T4467
Differential Revision: https://phabricator.kde.org/D7359