Defines the DecorationButton type in DecorationOptions (needs to be
re-defined due to QML limitations) and exports the buttons as a
QList<int> property (again QML limitations).
All QML files are switched from string to the new enum type.
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++.