ticket:8464: Initial API proposal.

git-svn-id: http://source.icu-project.org/repos/icu/icu@34637 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Integrated Markus' comments.

git-svn-id: http://source.icu-project.org/repos/icu/icu@34638 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Implement Markus' proposal to change reldates.h -> realdatefmt.h

git-svn-id: http://source.icu-project.org/repos/icu/icu@34639 251d0590-4201-4cf1-90de-194747b24ca1