Update list of notes

[korelstar]

We should frequently update the list of notes, which is shown in the app navigation on the left hand side. This is useful if you are changing your notes on another device in parallel or someone else changes your shared notes.

The related feature-request #310 is about updating the currently opened note (including content), while this is about updating the list of notes (without content). Hence, this will not be fixed by #331.

This is not just implementing a timer and refresh the list, but some concurrency issues have to be considered. E.g. the refresh must not show outdated data (which can happen if changing some metadata is faster than loading the whole list of notes). Therefore, a rework of the save process (JavaScript) may be required.