feat(datepicker): add viewChanged output to calendar (#13759)

[fhuschle]

Adds viewChanged output to calendar so you can react if the view changed between 'month' | 'year' | 'multi-year'.
Fixes #13759.

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[fhuschle]

@googlebot I signed it!

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[mmalerba]

If we emit here, the event will come before the UI has actually updated. I think people would expect this event to come after the view has updated. I think a Promise.resolve().then() should make this emit after, if not maybe a setTimeout(). Please also add a test that verifies this

[crisbeto]

Event emitters have an isAsync flag which we could use here? E.g. new EventEmitter(true).

[mmalerba]

@fhuschle can you try removing the setTimeout and passing true when constructing the EventEmitter? If that works we should use it.

[fhuschle]

That seems to work the same way.

[mmalerba]

LGTM

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