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/grid.h

@@ -302,7 +302,7 @@ public:
     // version only checks that the event has no modifiers. The derived
     // classes are supposed to do "if ( base::IsAcceptedKey() && ... )" in
     // their IsAcceptedKey() implementation, although, of course, it is not a
-    // mandatory requirment.
+    // mandatory requirement.
     //
     // NB: if the key is F2 (special), editing will always start and this
     //     method will not be called at all (but StartingKey() will)