Autosave

[rimesc]

Currently, the content of a note is only saved when toggling the editor pane visibility. This doesn't make as much sense now that the editor pane doesn't completely replace the rendering, so it's reasonable to leave it permanently visible.

Instead, the current content should be saved on the following events:

• when navigating away from a particular note
• when closing the application
• when editing a note (at most every few seconds)

[rimesc]

Fixed by a108624d49a987ff9c6708f096ae930a3e000630.