Add "Add Model's Selected Objects to ..." and "drag-select" to Process Flow membership

[guiroehe]

Interface consistency is important to facilitate users' work. 

When working with groups, the membership dropdown menu offers the option "Add Model's Selected Objects to Group". It also allows one to select multiple objects from the members list using mousedown-and-drag.

The same handy functionalities are not available in the interface for process-flow membership. Selecting multiple objects, one at a time by clicking is very inefficient. Having a more consistent interface increases software usability and should ease code maintenance.

Thank you.

P.S. One can always create a script for that, of course, but those are the basic things one should expect on a good interface.