When extra style bits are set with the call to
wxPropertyGridManager::SetExtraStyle(), only those which are relevant
to wxPropertyGrid should be passed to the underlying property grid object.
Because it can happen that not all extra style bits of the underlying
wxPropertyGrid have been effectively changed by call to SetExtraStyle()
(e.g. wxPG_EX_NATIVE_DOUBLE_BUFFERING), we have to get the actual style
bits prior to storing them.
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