Cleanup TODO

I want to track here a few areas i want to clean up as soon as 1.3.0 will be out (at least, the beta!)

1) TrackModel: do we need this class? Can we make it so general it does not need references to LayerTrack and LayerMap?

2) AboudDialog: can we use FreeTrackDialog as parent?

3) ConfigDialog: can we use FreeTrackDialog as parent?

4) FreeTrack: can we get rid of the StackedWidget and simply use the main content here?

5) MainContent: do we really need this class? Why not move all into FreeTrack and simplify a bit?