Instances of the showdesktop plugin in latest SVN panel makes the panel higher
(horizontal panels - haven't tested in vertical panels [1]) than configured in
the panel settings dialog.

[1]: I have just tried to make a new vertical panel, and the panel crashed when
I tried to make it vertical - will fill separate bug, if it isn't the same as
the last one I filled.

Reproducible: Always
Steps to Reproduce:
1. Add a new panel.
2. Add an instance of the showdesktop plugin to the new panel.
Actual Results:  
The panel size gets bigger than configured in the panel settings dialog.

Expected Results:  
The showdesktop plugin should respect the size configured by users.