Add new 'get_undershoot' method (amend 'draw_handle_textbox_positions')
Add new 'get_undershoot' method (amend 'draw_handle_textbox_positions')

This method gets called by 'structfun' in
'draw_handle_textbox_positions'.

It's used to get an array of all the objects's vertical positions in
that structure.

StefanRickli/anp