Custom Symbol tracking

[LeonMatthes]

Currently the Untyped Model uses either the Symbols from the current environment or ALL symbols.
Both are usually not what you want. All symbols are way too many, especially in images that use symbols for anything other than code. But the environment usually contains only class names and not selectors, which is not enough.
So we should implement a custom tracking, that saves all symbols that are currently part of methods. This could be updated whenever a method is compiled.
Open questions:

• What about method deletions? How do we track those?