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/private/grid.h

@@ -140,7 +140,7 @@ private:
     int m_col;
 };
 
-// header control retreiving column information from the grid
+// header control retrieving column information from the grid
 class wxGridHeaderCtrl : public wxHeaderCtrl
 {
 public: