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.
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