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.
hmm... I don't like this. Anyway, I think in the implementation we have to do this anyway, as we don't know the exact type. So I'm still not sure about this.