Remove duplicate `eval_when` methods

A number of objects have an `eval_when` method that are effectively the same (with minor differences):
- this evaluates an expression to a boolean within an execution context (either menu or UI)
- sets a trait on an object to the result (or not, if it errors)
- catches errors and reports them (in _most_ cases)

This is similar, but subtly different from the `UI.eval_when` method in that:
- it does `setattr` rather than returning the result of the evaluation
- the context used may be something other than the UI context (in particular, it may be the editor's `_menu_context`)

These methods can probably be turned into a utility function that gets passed the expression, the context, the object and the trait and has just one standard reporting mechanism.