Fix processing of dialog box items on NS

Fix processing of dialog box items on NS

* src/nsmenu.m (pop_down_menu): Remove old hack and discard menu
items.
(ns_popup_dialog):
([EmacsDialogPanel initWithContentRect:styleMask:backing:defer:]):
([EmacsDialogPanel windowShouldClose:]):
([EmacsDialogPanel dealloc]):
([EmacsDialogPanel processdialog:]):
([EmacsDialogPanel addButton:value:row:]):
([EmacsDialogPanel addString:row:]):
([EmacsDialogPanel clicked:]):
([EmacsDialogPanel initFromContents:isQuestion:]):
([EmacsDialogPanel timeouthandler:]):
([EmacsDialogPanel runDialogAt:]): Use the regular menu item
machinery to parse dialog items instead of the incorrect ad-hoc
code used previously.

* src/nsterm.h: Update prototypes.