ICU-13473 Add a new target to the VS project files called CopyUnicodeHeaderFiles...

ICU-13473 Add a new target to the VS project files called CopyUnicodeHeaderFiles, which will copy all of the project header files to the common output location. This is much cleaner and far less error prone than having each header file manually listed in the VS project file. Merge to trunk.

X-SVN-Rev: 40969