Mark GtkTextIter as inout parameters

Applied on https://gitlab.gnome.org/GNOME/gitg/commits/wip/albfan/gtksourceview. Without that, you cannot retrieve the selected lines to stage them instead of all the file

staging lines on gitg

related commits that cause the problem:

• remove custom gtksourceview vapi with those functions patched gitg@bdba5b75
• changes to use those params without being refs gitg@fe4a8bf2