[ui5-date-picker] improvement suggestions from discussions

[pskelin]

Bug Description

Summarizing some previous discussion results, could be iterated further:

• input event to be fired with same semantic as change event, assumption is that no one is interested in the temporary value while it is being typed, but this leads to issues when this temporary value is set via API back to the component. This would be preferred instead of telling people not to use this event.
• value state should not be changed in onBeforeRendering, this is an anti pattern and also makes the component behave differently from openui5 (there the value is not changed when set from the API)
• add isoValue to the event.details so apps that are interested in the value as ISO date don't have to go through hoops to parse the user visible value (isoValue -> value for 3.0)
• don't use DOM as a state storage, model all state as properties and render via template (see DOM manipulation anti-pattern

cc: @vladitasev