FR: display logos instead of browser names (configurable)

[ymeine]

This idea comes from @benouat

Having logos has some advantages:

• better visual distinction
• possible common size for visual harmony

When hovering the icon, additional information could be displayed, like the name of the browser and its version (what is displayed in the current implementation actually).

However I would not impose that to the user, a configuration option should be added. Especially for those who have difficulties to differentiate/remember quickly the subtle differences between all the IE icons (even if in this specific case we could generate a new icon with the version number overlaid, but well, this is a bit more complex :p ).