Intelligent autohide in the panel

[Jannis Pohlmann]

Hey Nick (mostly),

I have been working on the intelligent autohide feature in the panel
on and off the last 2-3 days. I've got it working nicely now. I think
it may suffice as a first-cut implementation, despite not doing neat
hide/show animations yet like some of those docks out there. It already
covers a few odd cases (desktop window overlapping the panels for
instance), which I found during testing. 

The details of how it works are explained in the commit messages. Here
is my branch, would be lovely if you gave it some testing and merge it
if you're happy with the changes:

  http://git.xfce.org/users/jannis/xfce4-panel/log/?h=jannis/intelligent-hiding

Nick: Simon and I chose a slightly different path compared to the patch
you sent to me, wherein the "autohide" property is replaced with a new
"autohide-behavior" option (possible behaviors: never, intelligently,
always). We felt that made a little bit more sense, despite the obvious
xfconf migration trouble (see the branch for that though).

I can make additional changes tomorrow, after that I'll be away at a
music festival until Sunday.

Cheers,

  - Jannis