Entry point must be set after skeletons not inputs

Previously the entry point for an activity had to be set after input
files, which in turn had to be set after skeleton files. Now both of the
values depend only on setting the skeleton files first.
Setting input files outside the activity factory
Adding Configurator

The first step in simplifying ActivtyRunner and pushing the
functionality inside composable LoaderInterface implementers.
Streamlining ActivityRunner interface