* applet.c: Add applet_do_popup_menu() and applet_popup_menu() to
support popping up of menu using Shift+F10 for buttons on the panel.

* basep_widget.c (base_widget_focus_return):
Update to forget previously focussed widget so that Ctrl+Tab followed
by Tab gives always gives focus to first button

* menu.c: Add menu_button_menu_popup() and menu_button_notify() to
support menu popup when menu icon receives focus.
Define callback for "clicked" to be menu_button_pressed so that
pressing Enter or Space on a focussed menu icon activates its menu.