Bugzilla

Work on the following section of the MEP spec

The hamburger button will appear and will show a condensed version of the hamburger menu which does not contain account options, new tab/window options, or bookmarks. 
The extension button will appear, and will show all extensions, even those that are pinned in a normal window. Any pinned extension buttons will not show in the toolbar. Within the extension menu, the option to pin extension buttons will be unavailable. 
The download button will be hidden by default, but will appear when there are downloads to view in the current session (where session is defined as the entire Firefox session including web apps, not the web app session alone. This should be what happens if we do no work.)

Add a singleton module that will be initialized in browser.init to setup all taskbar tabs related browser UI.