feat(ui5-datepicker): set value attribute through date object

[unazko]

On initialization of ui5-datepicker its 'value' attribute
string value should be formatted according to the used locale
and provided 'format-pattern' attribute.

Fixes #1503

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[vladitasev]

Great job!!
Works exactly as I would expect it.

Let's change the API a bit. Remove the setUTC* function and add a new function called formatValue for datepicker and timepicker. This function will just take a value and return a formatted value. Then the user will set this value through the official API. This will enable users to use their framework of choice's binding syntax. As we've done it currently, they can't call setUTCValue from their templates. But they can call formatValue to process whatever comes from their database, assign it to their state, and bind it to their templates.

TODO:

• Create formatValue function
• Document it with @public so that it starts appearing in the help
• Document how it works: you pass it a JS date object, and it returns a string, formatted according to the properties of the date/time/datetime picker instance
• Let this function be pure. Don't change any state of the component in it. When the user sets the real value property with the result of the new function, only then the valueState will be changed naturally.
• Update the samples