fixed broken `ViewportQuad` test
Merge pull request #496 from gbeatty/ViewportQuad

fixed broken `ViewportQuad` test
Ongoing MVVM refactoring

1. Add JulianDate.equals function
2. Make Animation destroyable.
3. Get rid of `ButtonViewModel.enabled` since it's redundant with `Command.canExecute`.
Merge remote-tracking branch 'origin/master' into playbackTweaks