Update DateTimeField.java

Factory method for the hours TextField.
No functional change. Added some todos in JavaDoc to mark those classes
and methods that can be replaced in Wicket 8 with standard Java 8
classes and methods.
changed to diamond operator
Merge branch 'master' of https://github.com/astrapi69/wicket into pr-80-master
Moved Wicket 8 todos aout of JavaDoc sections.