RegisterBusSubscribersCompilerPass: Add support for parameter return values

When I decided to use a parameter for my projector classes I received this exception:
```
ReflectionException: Class %my_parameter% does not exist
```
This PR solves this by asking for the parameter first before doing the reflection.

I saw that there is a test file for this. But before writing a test (I tested it manually with a parameter string and a valid class string) I'd rather check if this has a chance to get merged.