Fix problem with deleted remote files

Being deleted both locally and remotely meant a crash with
Dropbox backend. Add some null checks to prevent it.

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>
Fix list properties being multiplied like hell

This would cause the file to grow and grow. Now replaces
all sort/style tags when changed.

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>
Sync OrgSynchronizers in the background

Add a broadcast receiver that schedules regular background
synchronizations with the new backends. This is run on boot and
onCreate in the main activity. If none are enabled, then
the schedule is removed.

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>