ScreenPool: Ensure that primaryScreenChanged is emitted always after screenAdded

On Wayland, the primary screen notification and
QGuiApplication::screenAdded signals are desync. The primary output
watcher addresses that by emitting its primaryOutputNameChanged() signal
when the corresponding QGuiApplication::screenAdded() signal is emitted.

However, since the primary output watcher processes the screenAdded
signal before ScreenPool, it can emit the
ScreenPool::primaryScreenChanged signal before ScreenPool::screenAdded
signal that can confuse the ShellCorona.