[BUG #7063] changed the interface between migrator.py and patch.py to 'execfile'

- the concern with the old 'import patch' statement is that in case the migrator
  runs over multiple versions with patch.py files, the 'import' might not work
  for the second and all further files, as Python doesn't re-evaluate an
  imported module if it thinks it has already seen it
- so even with extending sys.path, the first patch.py might be checked over and
  over again, just to realize that this particular file has not changed,
  and never trying to re-import with the changed sys.path
- the new approach should make sure that for every migrated version its specific
  patch.py is used
[BUG #7063] removed unused code
[BUG #7139] removed duplicate reference to 'finally' clause
[BUG #7063] more toJS() adaptions for CST in treegenerator_3