This patch adds a LineBreaker class to Minikin, which will be used for
computing line breaks in StaticLayout. The version in this patch
contains basically the same functionality that existed before, but
hopefully better performance and an interface that's suitable for more
sophisticated paragraph layout.

Note that this version contains a high quality strategy, which mostly
works but doesn't respect varying line width.

Change-Id: I02485d58b1e52856296a72cdd4efd963bc572933