Wrote some own functions to hande pkg-config includes.
now Makefile generation will stop if xmms2-client-cpp.pc is missing
Now, compilation should work, even if xmms2-client-cpp development files
where installed after qmake das called for the first time.
also enabled adding files in the playlistview contextmenu
Use the new PixmapSlider class which is based on Qt's AbstractSlider
class for sliders. This should also fix some offset problems in
PositionSlider which appeared after the introduction of the new Button
class
