Bug 968156 - Store multiple icon references in app_window and places. r=...

...alive
The typical case is that the icons wont define their size, each icon can also be defined for multiple sizes which this format is particularly suitable to determine.

francisco is doing the follow up work to pick the best icon in https://bugzilla.mozilla.org/show_bug.cgi?id=1042027, we went back and forward a few times and it turned out this was the best way