[CMake] Factorized previous content in the examples cake file into a new, reusable include file.
Made the IPlugProjects folder a conditional include so that we can populate these with our new project and create new projects the same way we do with the examples. If the folder does not exists, then no added custom projects will appear in the complete build project.
typically the content structure of the IPlugProjects folder will be the same that examples one except that it will specify the user custom projects subfolders.
A new macro called create_iplug_project(name) should be used for your custom project (for now only an alias on the create_example(name) macro).