feat(ui5-date-picker): introduce value-state-change event

[hinzzx]

Currently in our <ui5-date-picker> component we have internal validation regarding value-state, which was setting up ValueState.Error/ValueState.None whenever the value in the component was invalid/valid.

The problem here is that when an app developer sets the value-state property explicitly it was being overwritten by the control, which is not always the expected behaviour.

To address this problem we are introducing the value-state-change event, which is fired before the internal validation/invalidation of the component. It is preventable, so when the app developer wants to take care of the state management of the component, they just prevent the event.

Behaviour with event prevented, and explicitly set valueState property to Error:

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Behaviour without event being prevented, and explicitly set valueState property to Error:

Fixes: #8005

[ilhan007]

related to: #7444

[ilhan007]

Everything good so far, the only thing I would like to discuss briefly next week is if we have to rather update the this.valueState before firing the event and later revert its value if the event is prevented. Or, to keep it like it is. I think in other components we have done the first, but still worth discussing it briefly.

[ilhan007]

Hi colleagues, @hinzzx as I mentioned previously, I wanted to discuss one detail before enforcing it as a review comment.
After brief discussion, we think it's more consistent to update the valueState property before firing the event, so that accessing this.valueState in the handler would return the new/updated value of the prop. And, afterwards, if the event turns out to be prevented - we revert the valueState prop to its previous value.

[ilhan007]

Thank you! I don't have any further comments, I will leave to @tsanislavgatev to also review/test/approve/merge the change.