Foundation Classes, Strings - add EmptyString() for Ascii/Extended and use it everywh...

Foundation Classes, Strings - add EmptyString() for Ascii/Extended and use it everywhere (#788)

Introduce TCollection_AsciiString::EmptyString() and TCollection_ExtendedString::EmptyString()
  and update headers.
Replace many local/static empty string instances and default-constructed
  temporaries with the shared EmptyString() reference across ApplicationFramework, DataExchange,
  Visualization and other modules.
Remove several unused local empty-key/static variables and
  adjust default parameters and return paths to use the centralized empty-string accessors.
Add missing #include <TCollection_AsciiString.hxx> to several headers that use/forward TCollection_AsciiString.