Prevent changing the primary screen when it doesn't have a view yet
Review Request #125693 - Created Oct. 18, 2015 and submitted
When a new primary output gets connected, it's possible that
primaryOutputChanged() is called before addOutput(). When addOutput()
was called afterwards, this led to two DesktopViews on the same
screen. This raised an assert in screenInvariants().
Solve this by not doing anything in primaryOutputChanged() when the new
primary output is not used for a DesktopView yet.
Plasmashell doesn't hit an assert in screenInvariants() after addOutput() when connecting a new screen set as primary screen.
(That doesn't mean it works, but at least there's a bug less)