Added documentation target for each plugin. If "Documentation" folder contains an html folder, the build system only copies this folder to the shared doc folder in the build directory instead of compiling the doc via doxygen.

git-svn-id: http://www.openflipper.org/svnrepo/OpenFlipper/branches/Free@9366 383ad7c9-94d9-4d36-a494-682f7c89f535