DropMessage refactoring: Don't use generic-parametered DropMessages

but ModelObjects as `data` field that shall be casted to specific target types afterwards.

Perhaps a reasonable way to get rid of the template-bracket-frenzy that's spread all over the code.
Please review and critizize - I haven't spent so much time in semantically walking through all the changes.

Resolves #199 once merged.
Well, `register()` only gets called during Qabel's startup routines, right?
And..that small 'performance' advantage of letting the compiler check for the given class to be some sort of ModelObject does imho not 
meet the advantage of not having generics and all its syntactical ugliness everywhere.