Width and padding improvements for the editor

[tecc]

Adds a bunch of editor improvements mainly relating to the use of screen space.

This PR:

• Adds an "as wide as possible" width which uses as much screen space as reasonably possible whilst still looking nice. Fixes Better ultrawide support #167.
• Moves editor width settings to its own button, aptly titled "Editor width". It replaces toggling between widths with setting specific widths (along with highlighting which one is active for the note).
• Improves sizing on smaller screens by using previously unused screen space dedicated to padding.

See #167 for more information and screenshots.