GtkMenuShell is a special kind of abstract
container that provides its descendants GtkMenu
and GtkMenuBar with methods and signals that are
appropriate to typical menu behaviour.
There are three keybindings set up here which are carried down to
the inheriting classes. These are as follows:
Escape cancels an existing popup menu
Return activates the current selection
Space also activates the current selected item
The activate-current signal has not been documented
here because any attempt to harness it beyond these bindings results
in abort. The same applies to the move-current
signal, implemented here and given keybindings in both
GtkMenu and GtkMenuBar.
Note that GtkMenuShell is an abstract base class,
and can not be constructed directly.