Generating import statements for newly added implementation classes

[Jörg Kienzle]

When Java code for a model that contains implementation classes is generated for the first time, the list of "import" statements (one for each implementation class) is correctly generated. However, when new implementation classes are added to the model, no additional import statements are added to that list.

[Matthias Schoettle]

Instead of protecting all imports, only potential user imports should be protected. I.e., by default there are no user imports.

However, there is no support for any other user modifications right now. They will be erased upon re-generation.

[Matthias Schoettle]

Resolves #465: Moves required imports out of protected import block (for user imports).

→ <<cset 1fa7f867df29>>