Large server side refactoring for DateField

Large server side refactoring for DateField

Changes summarized:
 - removed invalid implementation of isEmpty(). Can't work with validation/buffering. Added in #4311, fixes #5927
 - removed invalid overridden implementation of valueChange. Added in #5810 and #5277.
 - moved a lot of flag updates and other internal field updates to setInternalValue. Also tasks removed from valueChange(), now possible since #6002 is fixed.
 - n non-reported bugs fixed (at least some related to read buffering)
 - m regressions caused just a moment before large milestone?

Trivial minor enhancements to improve maintainability:
 - removed obsolete toString() method (same as in AbstractField)
 - added <?> to getType() method as in super implementation, fixes a compilation warning
 - decreased visibility of setValue(Object, boolean), should be exposed to extending classes only - probably opened accidentally
 - removed obsolete implementation of setValue(Object), did exactly the same thing as the parent
 - javadoc typo

Sorry for a large patch. Don't like these either. I just don't see how this could have been fixed in smaller patches (except for the trivial fixes).

svn changeset:16797/svn branch:6.5