Support non-restricted enum attributes

[jonathan-meier]

Consider the following simple rule attribute:

@Enum("Val1", "Val2", "Val3", restricted = false)
attr enumVal = "Val1"

CityEngine provides a drop-down menu for this enumeration with entries Val1, Val2 and Val3, but also allows to set an arbitrary value not in this list.

Serlio treats both restricted and non-restricted enums as restricted.

Task:

• Support non-restricted enum attributes, such that a user can set arbitrary values for these attributes

Note: A simple solution would be to ignore the enum annotation on non-restricted enum attributes and simply provide an input field. This way, the user would be able to set an arbitrary value, however, the enumeration options would not be visible in the UI anymore.