Improve user experience on slower devices

[korelstar]

I've tested the app on my old Samsung Galaxy Gio (from 2011) and found some bottlenecks: Especially, loading the list of notes from the database and the synchronization are very slow.

Loading the list of notes from the database

• Introduce database index to speed up sorting
• Maybe it could be helpful to save the excerpt in a separate column (with index) and then do not load the full note (i.e. load it only without the actual text), when only the list of notes is required.

Synchronization

• Synchronization takes very much time, because always all notes are transferred and compared. This could be improved, but requires changes on the server site (e.g. check if the ETag of the notes file has changed).

[stefan-niedermann]

@korelstar the synchronisation part has already been done, right? Maybe we than should drop this paragraph while the other proposals still make sense.

[korelstar]

Yep, you're right. I've updated the text and introduced check-marks to track the progress.

[stefan-niedermann]

Follow-Up in #528.