Fixing broken gegl_buffer_set() call in libmypaint-gegl.
Use ${includedir} as the base include dir, without the library name.

This would force devs to #include "libmypaint/mypaint-brush.h" instead
of #include "mypaint-brush.h".
Similarly for #include "libmypaint-gegl/mypaint-gegl-surface.h"
This is longer, but clearer for third party developers.
Reorganizing the tree to correspond to the libraries' organization.