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/selstore.h

@@ -57,7 +57,7 @@ public:
     // several items at once, return true if any of them were selected
     bool OnItemsDeleted(unsigned item, unsigned numItems);
 
-    // select one item, use SelectRange() insted if possible!
+    // select one item, use SelectRange() instead if possible!
     //
     // returns true if the items selection really changed
     bool SelectItem(unsigned item, bool select = true);