Make update checker independant from Qt application

This makes sense as:

• It is more of a core feature, only controlled by the GUI and using it to give the user some feedback.
• It needs to be fully tested without necessarily needing to start a Qt event loop.
• It could be runnable by the command line!

So let's move the logic out of application.py (typically refresh_update_status()) to put it for instance in updater.py. Then make update commands available from the commandline either through the Controller or directly calling the AppUpdater.

Also note that the periodic update check is done by the SynchronizerThread maybe this should be reviewed.