Various auxiliary updates.

- Link to GoTools (for no reason so far)
- Create a proper ownership structure of `ObjectSet`.
- Prepare for multithreaded loading of patches.

Right now, the idea for making new objects is this:

1. Call objectSet->addPatchFromWhatever(...) from a separate thread.
2. This will construct the display object, which will load the vertices
   and faces in normal (non-GPU) memory.
3. The ObjectSet will emit a requestInitialization signal. The GLWidget
   should listen to this signel.
4. When the GLWidget gets an initialization request, it should lock itself
   (this may not be necessary), make current the OpenGL context, and
   call the display object's init() method. This will create the OpenGL
   buffer objects, and make sure they belong to the right context. Finally,
   unlock the GLWidget.
5. Back in the ObjectSet, ensure that the object has been properly initialized.
   If not, show an error message, delete the object and return.
6. Then, lock the ObjectSet, update the tree accordingly, and unlock.
7. At this point, emit an update signal to the TreeView and GLWidget.
   (Not done yet.)

In the GLWidget's OpenGL context-sensitive methods, it's also necessary to
lock the GLWidget. Maybe.
Update signals from ObjectSet
Threaded adding of objects (appears to) work
Started work on documentation.
Use screen size for resizing factors
Varia

- Properly free memory.
- Support refinement in `DispObject`.
- Stress test (passes quite easily).
Better line rendering.
We don't need separate matrices any more.
Drawing of axes cross.
Even better axes drawing.
More version please
Better panning sensitivity
Fix depth of axes
Automatic bounding spheres
Better selection (not yet integrated)