apps/graphics/twm4nx: CWindow: Add a global function that can be used to determine the minimum width of a window. If the window is set to a width smaller than this, then toolbar items will overlap and the toolbar will be corrupted. This is a global function (rather than a method of CWindow) because in some cases, this minimum width must be known before the window is created.
apps/graphics/twm4nx: CMenus now uses CWindows, rather that raw NX windows. This rippled through to many other things due to conflicts between NUL-terminated C strings and NXWidgets::CNxStrings.
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