Python: Move dataflow call-graph to new qll file

Seems like all other languages use a file called `DataFlowDispatch`. I
want to introduce a setup where we have (old) points-to based approach
in one file, and can develop a type-tracking based approach in another
file, so that's the reason for the naming differing slightly.

For which predicates go in which files, I have taken mostly inspiration
from C# and Ruby.

Author: RasmusWL