* scheme (protocol) is always contained in path variables passed to JavaFileStorage implementors
* file chooser improvements (internal browser displayed also in file chooser list e.g. when selecting an attachments, compatible with Solid Explorer content uris, removed OI stuff)
* started GDrive support
* 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