containment: emit screen geometry change correctly
A containment reports the screen geometry. A containment has a lastScreen member which is the index of the screen it should be on.
We used to track changes to the screen before a containment was assigned to a screen, except the screen geometry got removed in a0b8bb2a.
reactToScreenChanged is called whenever Containment::screen changes this is where a containment is bound to an physical screen. It's a better hook for announcing geometry changes rather than having consumers make this assumption.