Don't allow editing a variable in debug view if it's not editable

[david-fong]

Feature Request

The Debug View's Variables view allows double-clicking to open an editable field to edit the variable. But it allows it even when the variable is not editable. If you try to submit an assignment to such a variable, you get "-var-assign: Variable object is not editable". Maybe this is queryable with -var-show-attributes?