CommonFileDialogs are not extensible. #38
Description
CommonFileOpenDialog and CommonFileSaveDialog are both sealed and can't be extended.
CommonFileDialog's abstract methods are all internal and can't be extended.
IFileDialogCustomize is completely internal.
etc., etc.
This is a common thing for BCL classes (and obviously a legacy of the original source) -- but it would be nice to have these permissions opened up so that we can extend the code without having to compile from scratch.