change build process to install generated plugin documentation with cmake

Previously, cmake called genManual to generate any plugin api documentation directly into $INSTALL_PREFIX. This approach is not compatible with the CPack system used to generate NSIS and DEB packages, meant 'make package' had to be run as root, and did not include the documentation in the generated packages anyway!

Now, genManual generates the documentation into $BUILD_DIR/share/manual/plugins, and the cmake install command is then used to install them. This rectifies the above issues.