software-properties-kde UI changes

[Jonathan]

Hi,

I've committed some minor UI changes to software-properties-kde. It's nothing 
too major; I switched the program over from using the bare Qt file dialog to 
use the KDE file dialog instead. I also gave most of the buttons icons where it 
made sense. In addition, I disabled the "Remove..." button in the GPG handling 
tab when no GPG key is selected from the list.

Thanks for rocking the docs,
Jonathan Thomas