Removed Thread.Sleep, added enter event for login

- I removed the Thread.Sleep(10000) from the MainViewModel. I introduced
a Timer that manages the stream updates. This might have broken app
resuming, further testing is required.
- Changed LoadData to private loadData, to prevent issues with the
stopTimer. Updates should only be initialized from inside the
MainViewModel, to ensure that the stopTimer doesn't start multiple
requests
- Made MainViewModel disposable
-> streamUpdater is disposed properly
- Added key down event for the username and password textbox. When the
Enter key is pressed, the login is started.