Fix extracted lineno with nested calls (#1126)

Fix extracted lineno with nested calls (#1126)

When a gettext call had a nested function call on a new line, the
extract function would use that nested call's line number when
extracting the terms for the gettext call.

The reason is that we set the line number on any encounter of an opening
parenthesis after a gettext keyword. This does not work if either we
have a nested call, or our first term starts on a new line.

This commit fixes that by only setting the line number when we encounter
the first argument inside a gettext call.

Existing tests were adapted to work according to `xgettext` with regards
to the line numbers.

Fixes https://github.com/python-babel/babel/issues/1123