Add all gegl include paths to AM_CPPFLAGS

Necessary to move declarations into their own headers.
Move gegl.h declarations into their own headers.

gegl-init.c -> gegl-init.h
gegl-apply.c -> gegl-apply.h
gegl-processor.c -> process/gegl-processor.h
gegl-c.c -> gegl-c.h
gegl-node.c -> graph/gegl-node.h
misc -> gegl-operations-util.h

gegl-c.h is intentionally left out of the documentation
list because it contains only deprecated functions.

The functions in gegl-operations-util.h are still duplicate
definitions, but moving them to another file for now is
necessary to fix the ordering of the documentation html files.

There is a guard against directly including gegl-operations-util.h
because its future is uncertain and it shouldn't be included
directly even in internal code.