TrackContentWidget: fixed background tile pixmap updating

The background tile pixmap used to draw the background of
TrackContentWidgets was only updated on size changes but not on
changes of zoom level in according editor. Fix this by updating
background pixmaps in trackContainerView::setPixelsPerTact().

include/track.h

@@ -232,6 +232,9 @@ public:
 	trackContentWidget( trackView * _parent );
 	virtual ~trackContentWidget();
 
+	/*! \brief Updates the background tile pixmap. */
+	void updateBackground();
+
 	void addTCOView( trackContentObjectView * _tcov );
 	void removeTCOView( trackContentObjectView * _tcov );
 	void removeTCOView( int _tco_num )