Prevent from being partly offscreen

Prevent from being partly offscreen

Limit the AppSwitcher to the screen size by either downscaling or
scrolling.

We scale the icons down up from 96, 64, 48, 32 to 22 and start scrolling
if we still fail to fit on screen.

The thumbnail box is shifted to either left or right, when failing to
fit we scroll here to.
To prevent from being offscreen at the buttom we adjust the thumbnail
height to fit.

The old positioning logic is replaced with a ShellGenericContainer to
implement a custom allocation system.

https://bugzilla.gnome.org/show_bug.cgi?id=597983