Since #223 the notes on transactions are stored and not deleted when editing a transaction, but it would be nice if we can add or edit a note on a posting from the app.

Currently I have implemented a simple way to edit notes, but before opening a PR I wanted to know the developer opinion. I've added a button on each PostingRow to show a Dialog with the note. This allows to create/edit/delete a note in a posting.

I am no graphic designer, but if you think this will be a nice feature I can made a PR.