Font Icon setting, and setting infrastructure

Uses a setting to allow, disallow use of Font Icon.

This probably needs tweaked since the Font Icon is used anyway in
some places, and never used in other places. Possibly the option
should be "Use Font Icon for dropdowns" (or maybe that should always
be enforced.

But this is a start on the infrastructure needed for those decisions.

Tried to cut down on the needless repeition in Moodle theme settings
via some "convention over configuration" too. It can get a bit crazy once you 
have as many settings as Elegance.