UI Tester Refactor - Use utility function for creating handlers which call methods on the control of an editor

In many editor implementations we register handlers that simply call one of the editor's control's methods. This repeated code could be more neatly refactored by creating a utility function (e.g. `create_handler_for_control_method`).
This function would be toolkit agnostic and theoretically could sit in a higher level `_interaction_helpers` module (currently does not exist).

See comment: https://github.com/enthought/traitsui/pull/1320#discussion_r501510793