[issue40893] tkinter: integrate TkDND support

[E. Paine]

E. Paine <paineelisha at gmail.com> added the comment:

I think I have fixed the problem with the 'data' attribute (and related getlist_event method) by exposing two event data attributes. One of these is a 'data_raw' attribute intended for use with text-based drops and the other is a 'data_split' intended for file drops (see the new docs for details).

My original idea was to expose the 'tkapp.splitlist' method and just give them the content of what is now the 'data_raw' attribute. However, 
I decided that should be avoided and so settled on the current implementation of two new attributes.

Thanks everyone for the effort you have already put into this and I know that there is a lot added in this PR but I would really appreciate some reviews to get it to a more commit-ready state (there is inevitably lots to be improved - I also apologise for saying 'appreciate' in almost every message :-).

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<https://bugs.python.org/issue40893>
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