aTV setting/dialog to sign in to /out from MyPlex (begin merging #150)

improvements over #150:
- GUI/handling: supports cancel, failed login, less screens/clutter on
the way...
remaining issues that need look-into:
- username/password are sent in plain text to PlexConnect (local
network). Need encryption?
- username/password will end up in logfile (PlexConnect.log). Be
careful, when posting an issue with Myplex Sign In...
- when username/password dialog is switched, it fades through the
settings page. Why? How to avoid?