FMSLogo Code

Integrate changes from trunk, which were made to make porting to Unicode easier.

Fix some build breaks when compiling for Unicode.

Rework CStringNodeBuffer to clarify the difference between bytes and characters.
Although this currently makes no difference, it will help when porting FMSLogo to Unicode.

Remove a #ifdef WXPURE by replaceing MessageBox with nearly equivalent wxMessageBox.
This is not exactly the same because the original used the deprecated MB_ICONQUESTION, which wxWidgets doesn'tsupport.

Change fileload() and filesave() to accept a wxString& instead of char *.
This is part of a large set of changes to support Unicode.

Use wxFopen() instead of fopen().
This was done to help port FMSLogo to Unicode.

Merge in changes from trunk

Remove the "Null_Word != NIL" test in make_strnode_no_copy().
It's a bug if Null_Word is not initialized by the time make_strnode_no_copy is called, so it's sufficient to assert it.

Change the string header of NODE* from char* to unsigned short*
Although it's not obvious, this adds some type safety because the header was always treated as a reference count (unsigned short), not a char*.
This was done to help prepare the code for being ported to Unicode, as the size of the header won't change, even if the size of a character does.

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