the intent of this header has canged over time. now it is already
systematically included with ustring.hxx and the operator overload it
provide fit nicely there...

Just to be safe, since that include as been added to the api during the
3.5 timeframe and therefore is already in 'production'
the header remain and simply attempt to include ustring.hxx
but a warning is issued indicating that this header should not be used
anymore... in a couple of major release we will thenr emove it completely

All internal users of that header are converted.

Change-Id: I8934c55f089e29d78c0f5649b7c87b2ecf024bad
Reviewed-on: https://gerrit.libreoffice.org/634Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>