The Corona generally is responsible for managing which screen a
containment is on.
However ShellCorona doesn't know which screen the inner containment of
the sysem tray is on.

This patch makes the systemtray's inner container emit the
screenChanged() signal as appropriate for the container the screen is
on.

This is needed to fix a large regression seen with the notification
applet which cannot get the right screen area in a multiscreen set up.

I'd welcome better suggestions on how to solve this as this isn't ideal.

Moved notification applet between screens
Emitted a notification with notify-send
They were on the correct screen