Widgets: Improve extensibility of the Gallery widget and of media widgets generally.

* Introduce a `widget_{$id_base}_instance_schema` filter for plugins to add new properties to a media widget's instance schema.
* Pass all of a gallery widget's instance props to the gallery media frame, not just the ones that core supports.

See #32417, #41914.
Fixes #42285.

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git-svn-id: https://core.svn.wordpress.org/trunk@41785 1a063a9b-81f0-0310-95a4-ce76da25c4cd