Bug 873251 - [Metro] Update buttons and backgrounds of Firefox app bar. r=jwilde

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rename : browser/metro/theme/images/forward.png => browser/metro/theme/images/appbar-forward.png
rename : browser/metro/theme/images/reload.png => browser/metro/theme/images/appbar-reload.png
rename : browser/metro/theme/images/stop-hdpi.png => browser/metro/theme/images/appbar-stop.png