panel execute patch

[Benedikt Meurer]

Hello,

I've modified the panel to execute commands whenever the main loop is
idle instead of executing them directly on button click. This improves
the responsiveness of the GUI (you can even feel it on fast machines)
and should give a better user impression. I welcome everybody to test
it and send feedback.

I also added a note to the TODO list for the panel, that we should
move the exec_cmd stuff to libxfce4util and/or libxfcegui4 after 4.0,
so all xfce programs benefit from this.

Benedikt

-- 
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