x11: Implement popup surfaces

x11: Implement popup surfaces

Make them use o-r windows, and move
with their parent.

We do a sort-of ok job on stacking order
here - whenever the parent window gets a
ConfigureNotify, we just restack all popups
directly on top of their parent. This is good
enough to keep popups on top of their parent
while we drag it around, and it gets the popup
to disappear when raising another window on
top of the parent.