xfwm4: assisted tiling feature

[Björn Fischer]

Hi everyone!

with my newly aquired fork privileges I pushed a first roughly working
version of that feature into this branch:

https://gitlab.xfce.org/bjorn-fischer/xfwm-4-tiling/-/commits/assisted-tiling

This is far from being complete: It works with compositing ONLY as it
needs an rgba visual to display the grid. Most things are hardcoded and
the old style tiling by approaching the screen borders is disabled for
now. But I think, the patch set should transport what I have in mind
with "assisted tiling".

It works much like the python demo: When you drag a window and press
(and release) a digit key "1".."5", a grid is displayed as an overlay or
OSD to which the window snaps to. Once a grid has been used, you can
"re-use" that grid by clicking right mouse button during window dragging.

Cheers,
Björn