* Changed TestCacheSupervisor for easier use of the many callbacks
* Adapted tests for new callbacks
* GroupBaseActivity: Added sync menu command
* Preferences: Added option to enable/disable offline caching
* App: don't lock database when user wants to reload. This is done in PasswordActivity and should be done there after the password was filled into the pw field
* CheckDatabaseForChanges.cs: used when syncing a non-cached database