Enumeration parameter input by drop-down menu

[Francesco Casella]

The values of enumeration-type parameters have to be typed in manually, but unfortunately there is no indication of what are the legal values that can be used. Either one knows them by heart, or one needs to switch to the textual view, find out the type name, then look for its definition (hardly an easy task, in particular for beginners).

Enumeration types are used ubiquitously in the MSL, so we need to support this unless people are always happy with the defaults.

Suggestion 1: use a drop-down menu

Suggestion 2: make sure there is a way to remove any value and get back to the default (i.e., no modifier in the source code)

Suggestion 3: leave an option to input the value manually (e.g. as a complex expression)

Suggestion 4: we could do the same for booleans, though everybody knows the values true/false for booleans

[Adeel Asghar]

Done in r23278.

Suggestion 1: There is drop-down menu now.
Suggestion 2: there is an empty option in the drop-down that should be selected to clear the modifier.
Suggestion 3: input of complex expressions can be done.
Suggestion 4: drop-down menu for Booleans as well.