Largish changeset that unfortunately had to be all done before it was worth commiting

1. Move all the toolbar code from MainWindow to MainToolbar, as this is where it really belongs
2. No longer use a layout to position items in the toolbar but position them manually. In this case, this is less of a hack than adding invisible items to make things line up. It was alpos needed for bullet point 3.
3. Add a small sub toolbar that only appears when the currently playing track has CurrentTrackActionsCapability. For now it is only used for last.fm tracks, but it makes the "skip", "love" and "ban" actions very easilly available. This should also be usable for many other things.

It is not perfect yet as it needs some work on positioning of the actions if there are more or less than 3, and it really should also have a fade in/out animation.