Add missing wxABI_VERSION checks and version script entries.

Newly overridden virtual methods must be taken in wxABI_VERSION checks as
otherwise any program using the class deriving from the one containing this
method would reference the method in the vtbl of that class and so become
ABI-incompatible with the earlier shared library versions.

Also add wxABI_VERSION guard around the entire wxSimplebookXmlHandler class
added in 3.0.2.

include/wx/propgrid/props.h

@@ -316,7 +316,9 @@ public:
                                 const wxString& text,
                                 int argFlags = 0 ) const;
     virtual bool DoSetAttribute( const wxString& name, wxVariant& value );
+#if wxABI_VERSION >= 30003
     virtual wxVariant DoGetAttribute( const wxString& name ) const;
+#endif
     virtual bool ValidateValue( wxVariant& value,
                                 wxPGValidationInfo& validationInfo ) const;