Adding images from content library to project should notice if image is already imported and not prompt for overwrite when the file is identical

Adding images from content library to project should notice if image is already imported and not prompt for overwrite when the file is identical. The checking should be done in addFilesToProject function (modelnodeoperations.cpp), which should just return success without copying anything if identical file already exists in project. Identical means same name and content, but e.g. combination of name and last modified could be used for simplicity unless there is already a ready made file API for determining if file is exactly the same. (Either in Utils::FilePath, or QFileInfo)