Disable sync button if no connection

This moves the sync command text update code into the handler that runs
(currently) every minute to update the last sync time. This allows us to
update the sync button's color and text to reflect changes in internet
connection.

Also, decrease the refresh interval to half a minute to make updates
happen more frequently. This can probably be decreased even more but
usually it doesn't result in any changes.
Step video: Unify encoding access

This allows us to easily change which video encoding we prefer to play
without updating it in many different places.
OfflineGuides: Remove outdated TODOs