Add a edit_integer widget.

[andrewray]

• Refactor the bindings code so it can be overridden and customised (via an optional parameter to edit)
• Add new action Custom_skip similar to Custom except the user function returns a bool. If false is returned the action is halted and the message is propogated up. Specific use is so that focus can move out of the edit_integer widget on Left/Right when at the beginning/end of the text. Not 100% sure how well this interacts with the macro code.
• add is_valid_char method to edit
• Add example editint.