Adds a new design-time font property for when `&displayMap` is in use in
a keyboard, so that the web debugger can use that font as appropriate.
If the keyboard does not have a `&displayMap` store, then the DisplayMap
font property is disabled, and the web debugger will use the OSK Font
instead, as previously was the case.
Note that the font for `&displayMap` has to be installed on the keyboard
author's system in order for it to be served up.
Fixes: #12123