Smarter desktop applet placement

[Ambroz Bizjak]

> I've written some code to do resizing better.
Now it's more complete and can handle vertical resizing as well.
There are some problems, though:
- Proxy widgets may still be badly placed, as the initial size reported is
wrong. I was thinking that could be solved by waiting for their size to
change by itself and repositioning them, however there seems to be no way
to do that. The geometryChanged signal from Applet is not triggered when
the widget resizes itself.
- Panels are still not considered obstacles. The easiest way to fix that
would be to just to get the rectangle of free space and use it for
positioning. Because I really don't want to have any unmovable objects in
the working area, that would severely complicate the algorithm.

I'm attaching patches for trunk and also for 4.1 (resolution changing is
somehow broken in trunk so I tested in 4.1), and also the new version of
the demo app.
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