Change Firefox OS icon shape to a circle

This PR adds a second, round SVG icon which is the source for the newly exported, round Firefox OS icons.
Also, I've added some icon sizes that were missing ([Firefox OS icon guidelines](https://developer.mozilla.org/en-US/Apps/Build/Manifest#icons)).

I know that the Guidelines technically allow for square icons, but round ones just fit in a lot better with the rest of the system, especially after the redesign in 2.0.

By the way, kudos for the nicely done app!