wxDataObjectComposite and protected method GetObject(wxDataFormat const&)

[hartwigw]

wxDataObjectComposite's GetObject method should be made public. This allows the use of simple data object's methods.

Code and documentation patch appended

[VZ]

(In [63381]) Make wxDataObjectComposite::GetObject() public.

Allow retrieving individual wxDataObjects from wxDataObjectComposite and add
an example showing how can this be done to the documentation.

Closes #11692.

[vadz]

Thanks, I corrected the example somewhat as I don't think you can use wxDynamicCast with these classes (and it's unnecessary anyhow) and also to make it more wx-like (notably removed the unnecessary this-> and (void)).