Refactor manifest.json to match both Android Chrome and Opera

[phbernard]

When manifest.json has been implemented in RFG, only Android Chrome was using it. Now that Opera supports it too, it must be updated:

• Which browser takes what? (potential conflict regarding the 48x48 icon).
• Maybe change the name convention (current icons have "chrome" in their name).
• Generate the proper icon (eg. the 16x16 icon for Opera).

This issue was first reported here: https://github.com/RealFaviconGenerator/grunt-real-favicon/issues/23

[nhoizey]

I know Firefox is also working on this support.

Thanks @diox for the link: https://bugzilla.mozilla.org/show_bug.cgi?id=997779