Toolbar button

A toolbar button should be added to quickly open the sidebar to the tabstore section (as an alternative to the hotkey or to clicking the sidebar button).

If we're able to we should try to have the button only open the sidebar on the desktop version of firefox, but on the android version it should display a window similar to the normal menu in the sidebar. Android firefox has no sidebar, but unlike in the desktop version button popups are new tabs, which should allow for loading tabs on mobile.