Plastik Style Removal

[Sandro Giessl]

Hello k-c-d,

The widget style Plastik won't be ready for the release, and I don't have time 
to keep up with maintenance at the moment. Plastik lacks a few important 
things like dockwidget/MDI buttons, vertical tabs, and general polishing.
I would therefore like to move it to playground. So if there are no 
objections, I will do the move in two days from now.

Regards, Sandro