Allow IconThemes to fallback on another IconTheme instead of Silk

If you don't want your IconTheme to fallback on Silk, you need to provide the backwards compatibility section with variables that don't exist or leave them empty.

If you don't provide the compatibility section, all the icons will fallback on Silk.
The issue is that Icon Fonts are usually monochromatic and they usually fallback easier on another icon font set, instead of an image type IconTheme.

For example, Glyphicon icon theme would be better to fallback on FontAwesome, instead of Silk, etc.

Ideally, in the configuration of the IconTheme we could suggest on what icon theme to fallback.