[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 :)

PS 11-18: Rebase

Change-Id: I4a03a84539fd3ac1273f0d3503fe8fafcd16be26
Merge commit 'refs/changes/10/1510/18' of https://gerrit.omnirom.org/android_frameworks_base into kk-sam

Conflicts:
	packages/SystemUI/res/values/custom_strings.xml