Add enum support

[Matthias Schoettle]

Enums already exist in the metamodel, but cannot yet be created in the tool. Add enum support to make it possible to create them.

As discussed, enums should be created the following ways:

• Until a better touch gesture is found, enums are created by drawing a circle.
• Using the mouse, extend the tap-and-hold (creating a new class) such that a selection menu is offered to create a class, enum or implementation class.

[Franz-Philippe Garcia]

Resolved issue #73: Added EnumView, EnumController,...

Resolved issue #34: Added ImplementationClassView, ImplementationClassController,...

Resolved issue #54: Added associations between any classifiers

→ <<cset 880ae84d3652>>

[Matthias Schoettle]

References #73: Changes the processing of entered literal names to only have the first character in upper case instead of all.

→ <<cset 46fbdbe0974d>>