Basic icon overlays, eradicate all uses of `!important`.

This adds basic icon overlays that can appear before or after a font-awesome
`span`; use `.k-icon-overlay` and optionally `.k-icon-overlay-before` or
`.k-icon-overlay-after` (default is after).

Add rule weight using `body` and/or `html` instead of using `!important`; the
latter means something subtly different.