Add time picker and date picker controls

Date- and time pickers are essentials controls. They can be build by combining different Controls 2 components but it would be much more convenient to have one date picker and one time picker control out of the box.

The time picker should take a QTime property as input. The UI could be based on the Tumbler example (https://doc.qt.io/qt-5/qml-qtquick-controls2-tumbler.html) with automatic AM/PM/24 switching based on the current locale.

The date picker should take a QDate as input and could be very similar to the QDateEdit widget (http://doc.qt.io/qt-5/qdateedit.html#details) with a QCalendar popup (http://doc.qt.io/qt-5/qcalendarwidget.html#details).