Added a VTK actor to render the Avogadro scene

Added a new library for VTK integration in Avogadro, added a
vtkActor derived class to enable rendering in a VTK render window.

Change-Id: Ia39735f99915a03a9344f333dac94d77d67d764d
Merge remote-tracking branch 'master' into vtk-widget

Conflicts:
	avogadro/CMakeLists.txt

Change-Id: I64f4d89a934a9990240583ad160afaa3672a6b65
Merge topic 'vtk-widget' into master

bac38ca0 Merge remote-tracking branch 'master' into vtk-widget
bdbc7f00 Added a VTK actor to render the Avogadro scene