X-TOUCH : add load/save scene controls

It would be great to be able to recall a scene from XTOUCH, and save as well. There are many controls unused in XCTL mode that could be used for that, and it could avoid using a laptop or mobile device running Xair app just for this purpose. The goal of XTOUCH in XCTL mode is to have easy and direct control over the mixer, being able to load/save a scene is very useful for example in a shared space environment with several bands/projects.