Tidy Containment screen concepts

The concept of Containment::reactToScreenChange() and virtual Corona::screenForContainment don't make sense under the new neat design. We no longer want to swap screen indices dynamically when screens change. It's persistent unless there's an explicit user action.

Leftover from the old design that has lingered and now some core Plasma code is now built backwards with ShellCorona keeping it's own maps which can get out of sync.

We should do the following actions:

• rename lastScreen to screenIndex

• have a setScreenIndex that plasmashell calls explicitly

• drop Containment::reactToScreenChange

• Containment::screen() and all the things relating to screenGeometry should be killed. Screen information is semantically not a property of the containment, at most it's a property of the view. A panel doesn't have a different screenGeometry to a desktop and the architecture is very messy. This requires a bit of client porting. It may need to be phased out slowly.