Other: change how pkg-config includes are handeles

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

src/playlist/playlistview.cpp

@@ -188,7 +188,8 @@ PlaylistView::contextMenuEvent (QContextMenuEvent *e)
 
 	a = new QAction (tr ("Add file"), this);
 	a->setShortcut (tr ("Ctrl+F"));
-	a->setEnabled(false); // FIXME: Disabled for now
+	connect (a, SIGNAL (triggered ()),
+	         qobject_cast<PlaylistWidget *> (parent ()), SLOT (menuAddFile ()));
 	qm.addAction (a);
 
 	a = new QAction (tr ("Remove selected"), this);