Add KDatePickerPopup

This is a popup menu wrapping a KDatePicker as well as optionally a number of common shortcuts ("today", "tomorrow", etc).

This is 18 years old and existed as slightly diverged copies in at least four repositories (akonadi-contacts, calendarsupport, zanshin and skrooge), as well as in here as well, as part of KDateComboBox. Let's finally stop that and have a single shared version.

This starts with the apparently most evolved variant from calendarsupport, and refactors that and KDateComboBox to end up with a unified implementation between those two.