They're not needed anymore, and break my build directory from time to time.
If this commit breaks your compilation, clean your build directory (or at
least the plasma and kwin subdirectories).
The Plastik decoration buttons are provided by a declarative
extension plugin as the canvas drawing API would be needed to
draw these buttons. This should be changed with Qt 5.
But it's also a nice example to show how a QML based decoration
can have an extension in C++.
