Allow extracting deeply nested calls in Python

[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.

Fixes #1125 (meanwhile also fixes #1123)

[dylankiss]

During the refactor, the order of extraction was also changed, as you can see in this test:
https://github.com/python-babel/babel/pull/1127/files#diff-c74d633b5cd37350f5a10b2697475119ba1db4f541eeccec744f7d79ab99d6c1R437-R452

It is now the same as the extraction order of xgettext. Also the comments extraction was fixed to be the same as xgettext and apply to all gettext calls (also nested ones) on the same line.
Nested translator comment with nested gettext calls are also supported now, just like with xgettext.

e.g.

# NOTE: Main Comment
_("Hello %s",
    # NOTE: Nested Comment
    _("Nested Gettext")
)

Both terms would get their right comment extracted.