textview: Look up the drop location iter on the right coordinates on DND

textview: Look up the drop location iter on the right coordinates on DND

We are passing widget coordinates to gtk_text_view_window_to_buffer_coords()
which expects coordinates to be relative to the text window in this case.
This may result in drop coordinates being displaced if the side windows to
the top/left sides are visible and taking space, so the DnD indicator will
point to the wrong position.

This can be seen on gnome-builder and gedit when displaying line numbers.