Fix Icon's issues

1) Correct browser action icon is not displayed in the toolbar palette (i.e. customisation mode). Because js is not executed there. Also default "default_icon.png" is not rendered, and is irrelevant (tested on OSX).
My solution is to replace "default_icon.png" substring from "browserActionBadge.html" file with the correct one on the fly.

2) Image is not correctly centered inside button
My workaround is to center image relatively to an html tag, not a button.