Remove some outdated comments

Now that this is part of a demo, it's perfectly safe to make these
assumptions.
Move background color to be a window property

Rather than being hard-coded.
Rename windowProperties to windowAttributes

This is X's name for these. Window properties are something different.
Implement X properties

It has no users yet, so the implementation probably has some bugs.
Deduplicate some common sizing code
Shuffle some code around
Use damage regions for filling the background
Move to a root window, rather than a list of windows

Window trees may be supported, but we don't have reparentWindow yet, and
it's not tested.
Remove special background drawing code

This is handled by the root window now.
Remove code to damage regions on window add

Since the window hasn't been configured yet, this is dead code. When we
do add these back as window attributes maybe, we'll do it by calling
configureRequest.