Chainfield as FilteredSelectMultiple

[alex20465]

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This PR contains modifications to properly handle ModelMultipleChoiceField base on FilteredSelectMultiple widget as chainfield.

It contains also test examples with a book-store, please take a look.

Here is a screenshot example:

Stations is the chainfield, station-variants and station-routes are based on stations which is a ModelMultipleChoiceField with the FilteredSelectMultiple widget