0027838: Foundation Classes - support wchar_t* input within TCollection_AsciiString and TCollection_ExtendedString

TCollection_ExtendedString/TCollection_AsciiString description
has been updated to reflect usage of this classes for Unicode strings.

TCollection_ExtendedString now defines constructor taking wchar_t* (all platforms)
and method ::ToWideString() returning wchar_t* (Windows only).
TCollection_AsciiString now defines constructor taking wchar_t*.

TCollection_ExtendedString/TCollection_AsciiString now defines
auxiliary methods ::StartsWith() and ::EndsWith().

TCollection_ExtendedString internals has been updated to eliminate
duplicated code for converting between UTF-16 and UTF-8.

Code has been cleaned up from redundant explicit conversions to wchar_t*.
Global method OSD_OpenStream()/OSD_OpenFileBuf() have been replaced
by C++ template to eliminate copy-paste for different STL collections.

OSD_SharedLibrary now uses wide-char system API call LoadLibraryExW()
on Windows for consistency.

New macro Standard_UNUSED has been added for marking possibly unused functions and variables
(to suppress gcc/clang compiler warnings).

src/NCollection/NCollection_UtfString.hxx

@@ -96,11 +96,15 @@ public:
   NCollection_UtfString (const Standard_Utf32Char* theCopyUtf32,
                          const Standard_Integer    theLength = -1);
 
+#if !defined(_WIN32) || defined(_NATIVE_WCHAR_T_DEFINED) || (defined(_MSC_VER) && _MSC_VER >= 1900)
   //! Copy constructor from NULL-terminated wide UTF string.
   //! @param theCopyUtfWide NULL-terminated wide UTF string to copy
   //! @param theLength      the length limit in Unicode symbols (NOT bytes!)
+  //!
+  //! This constructor is undefined if Standard_WideChar is the same type as Standard_Utf16Char.
   NCollection_UtfString (const Standard_WideChar* theCopyUtfWide,
                          const Standard_Integer   theLength = -1);
+#endif
 
   //! Copy from NULL-terminated Unicode string.
   //! @param theStringUtf NULL-terminated Unicode string