Add icon to Firefox toolbar

Adds a list.it icon to the firefox toolbar that can be used to toggle the sidebar. The user can move it around and its location will persist.
Known bug: when disabling/uninstalling list.it the "listitButton" is still listed in the currentset of the toolbar. But this has no impact on the UI and is removed the next time the user customizes their toolbars. It has no actual impact on the browser.