Allow extracting (deeply) nested calls in Python and Javascript

[dylankiss]

Currently the Python extractor does not support deeply nested gettext calls (deeper than as a direct argument to the top-level gettext call).

e.g.

_("Hello %s", _("Person"))
_("Hello %s",
  random_function(", ".join([_("Person 1"), _("Person 2")])))

The extraction code was refactored quite a bit to simplify the flow and support this use-case.

Currently the Javascript extractor does not support nested gettext calls at all.

The extraction code was refactored a bit to resemble the Python code as much as possible and support this use-case.

Fixes https://github.com/python-babel/babel/issues/1125 (meanwhile also fixes https://github.com/python-babel/babel/issues/1123)