libgnome-panel: add popup menu helper function

When applets want to popup a menu for a widget they need to know the current position of the applet. Then they must compute the widget and menu anchors that are needed by gtk_menu_popup_at_widget() to align the widget and the menu.

To avoid that every applet needs to perform these steps we introduce a helper function that automatically computes the anchor parameters from the applet position and pops up the menu for the widget.