[QPA] Implement Screen on top of internal Screens API

Summary:
The test DontCrashUseractionsMenu (Waylandonly) found an issue in our
screen handling implementation in the QPA. The code exposed a short time
frame between the dummy screen getting destroyed and the first screen
being added. This could result in a crash of KWin.

There is actually no need to implement Screen on top of Wayland screen.
KWin has all the knowledge, so we can also base this on top of the
Screens API.

Advantages:
 * no delays due to Wayland roundtrips
 * handle screen getting removed (was a TODO)
 * handle resolution changes (was a TODO)

The new implementation has a disadvantage that it destroys and readds
all screens whenever something around the screen changes. This shouldn't
be an issue in practice as it's only for the internal QPA and thus only
affects KWin internal windows which is placed in global coordinates
anyway. If it turns out to be a problem we need to track better the
screen changes - so far those were not tracked at all.

Test Plan: Run a few unit tests which change screens

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8345