Add .gitignore file.
Add requirements.

This is preliminary only.
Cleanup: pep8ifying sources.

This changes have been applied to all python files using
https://github.com/htgoebel/pep8ify and the result has been checked.
Cleanup: Eliminate repeated code.
Cleanup: wrap long lines.
Cleanup some ugly code.
Fix GtkWarning: gtk_range_set_range: assertion 'min < max' failed.

Includes some clean-up.
Refactor: Use _geticon() wherever appropriate + new module _resources.
Refactor: Use loop instead of repeated calls.
Refactor duplicate code: seek_forward/backward.
Refactor duplicate code: stop/skip_forward/backward.
Wrap long line and re-indent.

I missed doing this earlier.
Cleanup: Use optparse instead of twisted.python.usage.
Cleanup: Do not test for __main__. This is always run as script.
Rearrange imports: Import twisted and upnp_inspector after arg-parsing.

Thus they get not activated if parsing the arguments fails or
usage or help is shown.
Refactor: Use loop instead of repeated calls.
Cleanup some code.
Print traceback on error.
Cleanup.
Add shortcut for "Quit" and convert the entry into an image menu item.