ticket:10968: Add UDISPCTX_TYPE_DISPLAY_LENGTH and values for it, implement support (C)

git-svn-id: http://source.icu-project.org/repos/icu/icu/trunk/source@36352 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10968: Add #ifndef U_HIDE_DRAFT_API for the new enum values

git-svn-id: http://source.icu-project.org/repos/icu/icu/trunk/source@36353 251d0590-4201-4cf1-90de-194747b24ca1