KISS Code cleanup (removed copy-paste). (...)

- Removed mentions to MainWindowViewModel, since ours doesn't have a Model like that. Hell, our window doesn't have a view model to begin with (each control has its own).
- Kept MainWindowViewModel.cs anyway, for reference purposes.
- Obsoleted StartSensor() & StopSensor() until we can use Franck's KinectSensorController (branch f-hands-tracker).
Bugfix: Can't Close when it's already closing. Makes sense.
UI: Added status bar, linked to KinectSensors.StatusChanged.