menu button: Improve accessibility

Rework the way we assign an accessible name to menu buttons,
to make sure we pick up a label, should the button contain
one, and only override the name with "Menu" as a fallback.

po/POTFILES.in

@@ -30,6 +30,7 @@ gtk/a11y/gtkcomboboxaccessible.c
 gtk/a11y/gtkentryaccessible.c
 gtk/a11y/gtkexpanderaccessible.c
 gtk/a11y/gtkimageaccessible.c
+gtk/a11y/gtkmenubuttonaccessible.c
 gtk/a11y/gtkmenuitemaccessible.c
 gtk/a11y/gtkrenderercellaccessible.c
 gtk/a11y/gtkscalebuttonaccessible.c