Sort prototype categories

Currently, the order of the prototype categories shown in the Model Element Library is arbitrary: It is defined by the startup of order of the Eclipse plugins contributing PrototypeProviders.

In this ticket, categories should be sorted lexicographically to ensure deterministic behavior.

Note: Prototypes themselves should not be sorted since their order can be defined when developing the respective PrototypeProviders (see e.g., org.fortiss.af3.component.ui.prototypes.PrototypeProvider).