balsa-mime-widget-text: Hide quote prefix

Determine the top and bottom locations of a cite-bar using gtk_text_view_get_line_yrange() instead of gtk_text_view_get_iter_location(). The former looks across the whole line, whereas the latter looks at only a single character; if it is one that has been made smaller to hide it, the location and height are not what we want.

Closes #61 (closed)