GtkAccelGroup Constructor
Although not a part of the GTK object hierarchy,
GtkAccelGroup can be constructed in the same way
as most GTK classes, i.e.
In most cases, the newly-constructed GtkAccelGroup
would be used by all the menus belonging to that toplevel window, and
should be added to it using
add_accel_group() .
It is not always necessary to create an accelerator group in this way.
The GtkMenu method
ensure_uline_accel_group() calls
the GtkAccelGroup used internally by the menu for
underline accelerators for the duration of the period that the menu is
popped up. If none is designated, a new accelerator group is created
and used.