Changes to better handler the user deleting track 0.
The changes are to ensure that:
1. If track 0 properties are shown when track 0 is deleted by the user, the property view is cleared.
2. The track 0 tree node is collapsed.
3. The track 0 channel views (but not master views) are cleared so that if track 0 is re-opened, these views are blank. The master views are part of the composition, and so are not deleted when track 0 is.