Initial extension manager dialog/view/model (with non-final layout)

Shows the listing from the Brackets extension registry when you choose File > Extension Manager... The current layout is more or less copied from the web app; this will change once we have the XD design.

Some notes:

* The dialog is factored into Dialog, View, and Model. This might seem overly factored, but my assumption is that we might want to host the extension listing in something other than a dialog eventually, so I wanted to keep the Dialog part as just a thin shell around the View, and have the View contain all the real functionality. The Model is also really simple right now, but I expect that over time we'll add filtering as well as interaction with the underlying extensions and installation functionality to the model.
* I went with "Mgr" instead of "Manager" just to avoid ReallyLongManagerNames. This is inconsistent with our other files, though, so I'm open to changing it.
* The template for the view isn't localized yet because I wanted to wait until we have the final design before doing so (since most of the strings to localize are little connectives like "by" and "on" that might not even be there in the final design).
* I created a SpecRunnerUtils method to create a mock container element that's offscreen, but ended up not using it. I still think it's useful, so I just left it in, but we can take it out.
* The large spinner was taken directly from the Topcoat repo. It's a PNG (unlike the existing SVG) because that was what's available. We can swap it out later for SVG if we need to.