Add possibility to change a note's collection

[zecakeh]

… aka move notes.

In practice: copy the item into another collection and delete the old one.

This fixes #112.

Tested on web Firefox Linux
Tested on native Android

[tasn]

I'd rather have it as an explicit move operation rather than just being able to change the collection and have this done, as it has non-obvious implications for now (the fact that it's deleted and a new one is created).

[zecakeh]

I created a new screen to move the notes, I don't know if that's what you had in mind.

[tasn]

Yeah, that's what I had in mind! :)