Use terminal-specific hooks to display popup dialogs.

Use terminal-specific hooks to display popup dialogs.
* termhooks.h (struct terminal): New field popup_dialog_hook.
* menu.c (emulate_dialog_with_menu): New function, refactored from ...
(Fx_popup_dialog): ... adjusted user.  Also remove old #if 0
code and use popup_dialog_hook.
* nsmenu.m (ns_popup_dialog): Make hook-compatible.
* nsterm.h (ns_popup_dialog): Adjust prototype.
* nsterm.m (ns_create_terminal):
* w32term.c (w32_create_terminal):
* xterm.c (x_create_terminal) [USE_X_TOOLKIT || USE_GTK]: Setup
popup_dialog_hook.