This to prevent compilation issues between runs with and without precompiled
headers. Also remove the headers from the rest of the code base as they are
not needed there anymore, although they do relatively little harm.
Uniscribe is sometimes producing different results compared to ICU, especially
when RTL and LTR content is mixed. Comparing the results to other programs
(like editors or web browsers) leads me to believe that the result are at least
not worse than ICU and possibly better.
