ticket:10107: LayoutEngine improvements

git-svn-id: http://source.icu-project.org/repos/icu/icu@33535 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10051: InitOnce and Mutex, work in progress.

git-svn-id: http://source.icu-project.org/repos/icu/icu@33536 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10107: merge of r33535 before 51.1.1 version # bump

git-svn-id: http://source.icu-project.org/repos/icu/icu@33537 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10107: ICU 51.1.1 version bump and nominal readme

git-svn-id: http://source.icu-project.org/repos/icu/icu@33538 251d0590-4201-4cf1-90de-194747b24ca1