feat(ui5-calendar): Declarative dates support added

[vladitasev]

Changes:

• new component added: ui5-date with a single value property of type String (formatted date)
• ui5-calendar has a new slot: dates that accepts instances of ui5-date
• The selectedDates property is removed from ui5-calendar and replaced with a getter and setter which mimic the old behavior entirely. get selectedDates returns an array of UTC timestamps and set selectedDates allows to pass such an array and build the dates as children of the calendar itself. This can be used as a utility and is used internally.
• The user can suppress the selected-dates-change event in order to prevent the default behavior of the ui5-calendar creating instances of ui5-date inside itself.
• The ui5-date-picker, ui5-daterange-picker and ui5-datetime-picker components have been slightly reworked to use the new API of ui5-calendar (although they would have worked with the compatibility getter/setter too).

BREAKING CHANGE:

• The property selectedDates is deprecated for ui5-calendar. Instead of setting selectedDates as an array of UTC timestamps, please provide instances of ui5-date as children of the ui5-calendar. Instead of reading the selectedDates property to find out which dates the user selected, please read the children of the ui5-calendar (instances of ui5-date with value property).
  Important: for backward compatibility there are the get selectedDates and set selectedDates getter and setter methods that mimic the old behavior entirely so your code can continue to work.

closes: #2528