Persist unsaved/scratch documents about rnote

[EverardNisse]

Today I learned that this wonderful key feature is still cooking in the oven... by forgetting to save a document :)

[MariusAlbrecht]

I like this feature. I also like the idea of widening the issue so that all tabs -regardless of whether they've been saved- will be restored on startup. However I don't like the idea of this always being enabled, as is the case with crash recovery. I think its to be expected that -by default- unsaved documents actually won't be saved.
I also think its to be expected that -by default- the program will start up without any opened tabs.
If the user actively chooses to not save documents and close the program then nothing (meaning both unsaved documents and open tabs) should be saved and later restored.
The only two reasons for recovery I see are

1. anything but the user causing a loss of data (e.g. a crash). In this case the user didnt consent to that loss.
2. an opt-in.

I think recovery should overall be handled similar to session restores in e.g. firefox (https://support.mozilla.org/en-US/kb/restore-previous-session), basically always trying to restore in irregular circumstances (crash e.g.) but requiring an opt-in for anything else.

[LeSnake04]

First of, sorry for the delay on document recovery, I got my first full time job last summer and was busy with other projects for the past months.

I have thought about this as well since someone suggested that on the matrix channel and realized when my PR is finished this feature should be pretty easy to implement since the recovery already does the caching part, and we just need to not clean up when close and maybe open the document automatically.
(Fun fact: During development of recovery I basically had this feature for a short time until I did the cleaning up function on closing)