I should not be able to enter illegal characters in Number fields

(User Story, To be Fixed , Priority: Medium, Test Status: No automated tests yet , Reported By Justin du Coeur, )
User Interface
Summary: Currently, if I enter "foo" into a Number field, the back-end validators catch it and reject it. That's not bad for a start, especially now that the UI for the error is decent, but we should really prevent it at the start.
That is, you shouldn't be able to enter non-numeric characters (aside from "-" at the beginning) into a Number field.
While we're at it, if the field is ExactlyOne Whole Number, you shouldn't be able to submit if it is empty.
Actually implementing this is a little tricky, mostly in getting the coupling right. It probably implies that every _propEditor should include classes that indicate its Collection and Type, so that those can be hooked by Gadgets. Have to think about how to add an "input filter" like this, though.