[FE] Order process variables list alphabetically

Summary
Order process variables list alphabetically in the modeler. When we have a lot of process variables it becomes complex to find easily the right one.
We can imagine some enhancements for a future version about this dialog: 

• Add Filtering capability on "name" column

• Add Ordering capability through the columns header 

Expected behaviour
List of process variables ordered alphabetically.

Current behaviour
Last created variable is displayed at the very bottom of the list