Adding header license to C++ files and removed some empty files

We have a MPL2.0 license. It is better to add for each of our files the license information (FSF recommendation). Point is, if someone finds one of our class usefull, having the whole repo to copy to get the license info is cumbersome. Since each file carry their own license info, it is way easier like that.

Having the author list of the single file in the header have, IMHO no sense at all: Git is there to perform this task. And 90% of the time, I personally do not add myself to it, either because I forget to do so, or because I do not care. I am not probably the only one since I've found some of our files with no license information whatsoever.

Having a consistent header license information also let us upgrade later on if we need to specify a copyright. I've just used the MPL2.0 license from Mozilla template for now. We can add very easily later on something like: Copyright (c) The Malia Rendering Framework development team.

Some light cleanup also was done removing some files that are empty.