Use classes to apply the 'editfont' preference

Use classes to apply the 'editfont' preference

This class can be re-used on non textarea inputs
(e.g. ContentEditable or <input>).

The 'default' preference can't be achieved in CSS alone, so
for non-textarea cases, use 'monospace' as is used in the majority of
browsers/OSes. In future a client script could compute the default
font of textarea and add it to the -default rule.

Bug: T145015
Change-Id: I5f0433e9cae6f692396537412c11b79b61db062f