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/univ/scrolbar.h

@@ -131,7 +131,7 @@ protected:
     virtual void DoDraw(wxControlRenderer *renderer) wxOVERRIDE;
     virtual wxBorder GetDefaultBorder() const wxOVERRIDE { return wxBORDER_NONE; }
 
-    // forces update of thumb's visual appearence (does nothing if m_dirty=false)
+    // forces update of thumb's visual appearance (does nothing if m_dirty=false)
     void UpdateThumb();
 
     // SetThumbPosition() helper