kdeui: DateTime widgets improve api and implementation, add tests

Sorry guys, meant to push this before Beta 2 but clean forgot, one
downside to git and feature branches is loosing track of things...

Update new KDateComboBox, KTimeComboBox and KDateTimeEdit widgets with
improved api and add unit tests. These classes are now ready for final
review. The KDateTimeEdit may need a couple more bug-fixes.

Remove new KDateEdit and KTimeEdit sub-classes as I decided their use
case was very limited and not properly understood.