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/encconv.h

@@ -96,7 +96,7 @@ class WXDLLIMPEXP_BASE wxEncodingConverter : public wxObject
             bool Convert(wchar_t* str) const { return Convert(str, str); }
 
             // Return equivalent(s) for given font that are used
-            // under given platform. wxPLATFORM_CURRENT means the plaform
+            // under given platform. wxPLATFORM_CURRENT means the platform
             // this binary was compiled for
             //
             // Examples: