This method opens the popup. In fact, it only calls the
predefined MUIA_Popstring_OpenHook and checks its return
value. In case of TRUE, the popup button object is disabled
as long as MUIA_Popstring_Toggle is unset.
If the toggle mode is enabled, using MUIA_Popstring_Open on
a currently opened popup will result in closing this popup
(i.e. calling the close hook) with a success value of FALSE.