Add view pane toggle icon in main UI

When first setting up Mailspring you are asked whether you’d like to activate a permanent view pane or not. While view pane on/off can also be toggled in /Preferences/Appearances it would be nice to have quick access icon on the main UI, similar to the “hide sidebar” one.
Not mission critical of course.

You can do this from “View” in the menu bar. So it is two clicks, and not one, but easier than to go to the preferences.