Implement a basic editor

[marph91]

Ref: https://discourse.joplinapp.org/t/joplin-sticky-notes-stick-your-joplin-notes-to-desktop/30711/14

Work steps:

1. Review other note taking applications (f. e. https://techviral.net/sticky-notes-alternatives, https://discourse.joplinapp.org/t/scratchpad-for-macos-system-tray/27529/30) and make a UI concept.
2. Check if there are existing open source implementations, we could base on.
3. Implement/integrate the editor
4. Verify it works properly with some test notes. I. e. formatting and syncing with Joplin. (What should happen if the note gets changed in Joplin and in the sticky note?)

Another obstacle is that the notes are currently saved as HTML. Making them editable would mean to save and edit them as markdown and convert to HTML for viewing only.