Eliminate Workspaces menu magic numbers.
A number of functions relating to the Workspaces menu assume that the
menu contains at least three entries: New, Destroy Last and Workspace1.
Because it is assumed that index 1 contains the Destroy Last entry and
index 2 contains the first workspace entry there are several occurrences
throughout workspace.c of magic numbers relating to those indices.
Replace the magic numbers with some #defines, making the code more
readable and allowing the menu to be expanded later.