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*

interface/wx/object.h

@@ -625,7 +625,7 @@ public:
 
     /**
         Conversion to a boolean expression (in a variant which is not
-        convertable to anything but a boolean expression).
+        convertible to anything but a boolean expression).
 
         If this class contains a valid pointer it will return @true, if it contains
         a @NULL pointer it will return @false.