Use the High DPI images from the firefox theme

This pull request fixes the tabs looking blurry on high dpi screens when the user has changed the layout.css.devPixelsPerPx value in about:config.

The @2x images were taken from the GNOME firefox theme, and added to this theme. Also, the CSS was updated with a media query that shows them.

Not also, that there are also images in the theme/chrome/messenger/tabmail folder that don't seem to be used in this theme, but i moved over the @2x versions of those as well just for consistenty (all the images with "inactive" in them arent used in the thunderbird theme currently)