Revert "[WIP] Multi-window"

This reverts commit a9022c7ba9a988152085f23bd635c6b1cdaa3f96.

Conflicts:
	packages/SystemUI/res/values/strings.xml
[WIP] Multi-window

This is a big work in progress, which currently has some bugs,
but begins to look okay.

You can already multitask a little bit, by using the
app switcher and long-pressing an icon, then select "Add to
Split View".
Last window touched gets the key focus.

Code is very ugly, I know. Don't merge it, don't use it
for now. Unless you want to make your code ugly.

Enjoy :)