Synfig-core file reorganization

It would be nice to do a file reorganization in the synfig-core sources. I would start with things like putting all the valuenodes and layers in subfolders using the same Makefile_insert technique already being used in synfig-studio.
After that, I would like to split other big files and so on.

Changes like this touch a lot of files but hopefully only the include paths and so on. Anyway I will try to do this in a quick manner every time: create a branch, do some reorganization and immediately do a pull request. Updating frequently would be advised.

The first pull request is already there:

It has been for ages in my TODO list. Good luck with it!