Consider creating a standalone Workbench package

`pyface.workbench` has long been deprecated but we are unable to remove it entirely from the codebase as MayaVi still uses it. We might not be able to find time to port MayaVi from being a Workbench-based application to being a Tasks-based application. Note that we made such an attempt earlier. Because of this, one option is to move all Workbench-related code pieces into a common package - tentatively called `etsworkbench`.

Note that we are considering other alternatives to this approach - one being that MayaVi simply vendorizes all relevant pieces of Workbench until it can move away from Workbench. Given the time/resource constraints, this might be the most feasible option.