Add full local export

[licaon-kter]

First use case: backup with Syncthing
How to: you have a note updated, the text file is updated too, Syncthing makes a backup
Primary thing to be done: have all the notes exported all the time

Second use case: sync between devices with Syncthing (multiple devices share a folder)
How to: on start (and periodically) the app loads (newer) notes from local
Primary thing: exported notes have their dates note1-UTC20161125123456in name as a simple way to keep track of last update

Yes, the multiple devices use case could get complex, but I'm thinking of 2 or 3 devices that have internet access most of the time.

This concerns mostly text files as exported now, I like them better as I can edit them on a PC too without any hassle. If you want them binary, as the app keeps them, it might be easier or harder, I don't know now.

[Kamuno]

Reminders and other relevant data should also be exportable.

[T-vK]

See #94