Calendar: Improve comment
Calendar: Rename multiple calendars demo to multiple months
Calendar: Sort and clean up demo index
Calendar: Fix demos after widget split
Calendar: Add buttons option
Datepicker: Add buttons option
Datepicker tests: Rework structure unit tests for buttons option
Datepicker: Remove outdated localization tests
Calendar: Fix wrong class in _createButtons method
Datepicker: Add option unit tests for buttons option
Datepicker: Fix multiple calendar styles (follow-up)
Datepicker: Remove duplicate demos
Calendar: Rename multiple calendars demo to multiple months (follow-up)
Calendar: Remove unneeded image
Calendar tests: Rewrite core unit tests based on datepicker tests

Remove duplicated core unit tests from datepicker tests.
Calendar tests: Add common unit tests
Calendar tests: Remove unneeded image
Calendar tests: Fix title
Calendar: Set selected property to null by default
Calendar tests: Rewrite method unit tests based on datepicker tests