Custom 2D pointer creation

[febret]

Implement

• IPointerFactory: has a createPointer/destroyPointer method. Add this to
  Pointer.h
• ImagePointerFactory/ImagePointer/ImagePointerRenderable add to omegalib, basic
  pointer factory for creating image-based pointers.
• Add Engine.setPointerFactory method. If set to NULL go back to standard pointer.
• Register Engine.setPointerFactory, ImagePointerFactory and (if needed) IPointerFactory
  to python.

Pointer factory can be added through a script or through config file using the
config-based module creation system:

modules:
{
   imagePointer: { class="ImagePointerFactory"; image="foo.png" }; 
}

or, if we implement #136

modules:
{
   ImagePointerFactory: { image="foo.png" }; 
}

NOTE ImagePointerFactory will need a static create method taking a module
name string to work with this. create will also take care of setting up stuff
based on the configuration entry.