ScreenPool as the source of truth of QScreen info

index desktop views by qscreen instead of id, making ScreenPool
the single source of truth for the mapping between screen names
and ids. This is less error prone and easier to consistency check
(if view->screenToFollow() is ever different to its has key it will
assert)
The whole logic of screen management is moved to ScreenPool.
ShellCorona will have to never call QGuiApp->screens, but only trust what ScreenPool it's telling it

Also adds an autotests on screenpool which makes a fake wayland server which sends screen added/removed/changed events