text view: Improve tag pointer tracking

text view: Improve tag pointer tracking

A problem that has been observed in polari is that links in tags
are clickable all the way into the margin. This problem is caused
by gtk_text_view_get_iter_at_position ignoring the return value
of pango_layout_xy_to_index. Instead, pass it back as a boolean
return value. This is technically an API break, but we've allowed
ourselves to change return types from void to gboolean before.