Multiwindow

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 :)

Cherrypicked from Omnirom