notice when the only screen changes

Summary:
in a corner case that is used a lot, the internal laptop screen
gets automatically disabled when an external screen is connected.
the only QScreen* available from the QGuiApp gets recycled for the
new screen and there is no signal this switch occurred.

To work around this, as all the view get an expose event when this happen,
monitor the rename of the desktopview's qscreen and manage this separatedly
in the shell.

BUG:373880

Test Plan:
connection and disconnecting an external screen to the laptop
both in the case of internal screen enabled or disabled

Reviewers: davidedmundson, #plasma

Reviewed By: davidedmundson, #plasma

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3777