Fix spelling in comments and documentation using codespell

Apply the utility from https://github.com/codespell-project/codespell/
to fix spelling issues in the headers under both include and interface
directories and add a file with a couple of exceptions.

The exact command line used was:

    $ codespell -w -I misc/scripts/codespell.ignore -i 3 in*

include/wx/generic/dcpsg.h

@@ -90,7 +90,7 @@ public:
 
     void PsPrint( const wxString& psdata );
 
-    // Overrridden for wxPrinterDC Impl
+    // Overridden for wxPrinterDC Impl
 
     virtual int GetResolution() const wxOVERRIDE;
     virtual wxRect GetPaperRect() const wxOVERRIDE;