Specify icons in buttons via CSS rather than <img> tag.
The CSS for icons is either in tundra.css (for the editor icons),
or in the customer's css file (for custom icons).
Toolbar buttons have an icon and text defined but you can control which shows up
via CSS rules (the text needs to show up if dijit_a11y is set). Haven't created
those rules yet though, so for now both icon and text show up.
Need to doublecheck a11y stuff; I think I might have broken it.
Refs #3455.
When MenuItem? is converted to extend Button it will inherit this method of
setting icons.