Transforming the <image> element causes it to become smaller, and thus
to not trigger click event if the button was clicked near the edge.
This patch makes us scale tab close button by instead changing its
padding which just makes the icon smaller during :active.
This fixes#526
Fixes#362
This adds scaling to:
Downloads button
Reveal file button within downloads panel
Urlbar informational icons
Extension menu button within extensions panel
Tab close button
