Add squares to "select all" on page and option to do same when searching

Gmail currently allows you to select a top checkbox which either checks all the emails on page or unchecks them depending on their current state. Additionally when filtering based on a search and you use that checkbox gmail gives you the option to “select all mail which matches” allowing you to do mass deletes of things like deal alerts, promotional emails etc which can build up over time. I’d like to be able to do it in mailspring and not have to go to gmail web to accomplish it.

(Originally posted by famewolf on GitHub.)