Add support for json notes import

[andiandi13]

NotallyX can export notes are json, and I realized it's pretty cool, because it saves all infos for each note (if it's pinned, modification date, color etc.)

But I don't see the point, because we can't import them back, the json code will be imported as raw text.

Database backups are not easily editable, so giving this option for people willing to keep their backups as individual json files that they can edit anytime outside of the app is a cool feature.