Add an additional visibility flag to the menubar implementations,
conditional on whether or not the menubar overlaps the window content.
(I.e for PUI but not Cocoa). This flag is linked to a new property
/sim/menubar/overlap-hide, which the renderer drives off the splash-
screen visibility.
Was only cleaning up the PUI menubar, not old bindings / names. This led to duplicate bindings, each time the GUI was reset, which can happen for various reasons.
Adapt FGPUIMenuBar to use new Locale module.
Adds support for separate "key" property for menu items, so keyboard hints
are no longer part of the item's label (and do not need to be part of the
translated text resources).
