Add USER properties to QDateEdit and QTimeEdit.

Add USER properties to QDateEdit and QTimeEdit.

Both classes had such components before, but there were issues with
the NOTIFY signal not being in the same class as the Q_PROPERTY.

This patch solves that problem by using a signal of a different name.

Task-number: QTBUG-15731
Change-Id: Ibc7ce4dba8a6b88c05d62a90e14d0101c5cd3082
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>