Whenever the popup receives a MUIM_Popstring_Open method,
this hook will be called. It will receive a pointer to itself in
register A0, a pointer to the complete popup object in A2 and
a pointer to a pointer (!) to the string object contained in
the popup object in A1.
When this hook returns TRUE, MUI assumes the popup was opened
succesfully and will disabled the popup button (as long as
MUIA_Popstring_Toggle is not set). Return FALSE to indicate
that something went wrong and the popup could not be opened.