Pin folders

The ability to pin a folder (or subfolder) from an account, so it shows as a top-level icon on the left-hand bar, when the bar is collapsed or expanded.

Currently, mail is sorted by server-side rules into subfolders: but when I open Mailbird, I don't see these subfolders: I need to manually expand the left bar and then open the appropriate top-level folder. There are only a few top-level icons, so it'd be great to be able to choose additional ones.