WIP: get dviewer_engine to compile
WIP: remove DVector, DMatrix and friends. Get DViewport to compile again
WIP: make dviewer-gtk compile again.
WIP: dviewer-gtk, fix some crashes.
WIP: fix some gsl/glib mix-ups

bugfix: DTrajectoryControl only end main_loop if there is one.
    Use same convention for alloc/free as glib's ref/unref
DTrajectoryControl: Implement asynchronous  orders with GMainLoop
DTrajectoryControl: Fix refcounting errors and pointers.
DTrajectoryControl: change DVectors to gsl_vectors
Make DViewport useful again
DTrajectoryControl: make set_current_position{,_axes} methods public