Simplify the creation of MCE views somewhat:

* Rename the `wp.mce.media` mixin (which was named too ambiguously) to `wp.mce.av`.
* Use the same technique for extending mixins for MCE base classes for views and their base.View property class
* `wp.mce.views.register()` should have default constructor logic if one is not passed.

Props avryl.
See #28458.


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