display message if project is empty and user tries to export it (closes #2091056)

git-svn-id: https://lmms.svn.sf.net/svnroot/lmms/trunk/lmms@1571 0778d3d1-df1d-0410-868b-ea421aaaa00d

include/track_container.h

@@ -60,10 +60,11 @@ public:
 	int countTracks( track::TrackTypes _tt = track::NumTrackTypes ) const;
 
 
-	virtual void updateAfterTrackAdd( void );
 	void addTrack( track * _track );
 	void removeTrack( track * _track );
 
+	virtual void updateAfterTrackAdd( void );
+
 	void clearAllTracks( void );
 
 	const trackList & tracks( void ) const
@@ -71,6 +72,8 @@ public:
 		return( m_tracks );
 	}
 
+	bool isEmpty( void ) const;
+
 	static const QString classNodeName( void )
 	{
 		return( "trackcontainer" );