groups: move all the logic of persistor saving from main.py to Base.

Should have been a part of fef3723 actually.
fix: tracebacks in 'group remove ...'

Solver needs the pkg_name->reason mapping function now.
fix: do not traceback on comps remove operations if proper pkg reasons can not be found.
groups: do not error out if group install/remove produces no RPM transaction.

The group semantics is wider than that now.