BUG: Fixed crash on scene close

Node pointers in the scene model can become invalid when the scene is closed.
If the node is looked up by node ID and pointer is retrieved from the scene then the retrieved pointer will always be valid.
The d->MRMLScene->GetNodeByID() lookup takes some extra time, but it is not very slow, because it just finds a string in a std::map<string,...>.
ENH: Allow displayable manager to be introspected using python

This is particularly useful when debugging issues.
ENH: Remove redundant markups displayable manager method

This commit remove the method "GetSliceNode" and instead make
use of the existing method "GetMRMLSliceNode"
STYLE: More explicit debug message in displayable manager group

The class name of the registered displayable manager is displayed.
BUG: Update CTK to fix "no active" camera warning

// --------
vtkLightBoxRendererManager - Fix warning related to missing active camera

This issue was occurring after (1) adding a markup and (2) registering
a custom Slicer layout having at least on Slice view.

It addresses the following warning:

  WorldToView: no active camera, cannot compute world to view, returning 0,0,0

The "no active camera" warning/issue is also discussed in Slicer issue
#1690 but note that this commit does NOT address that problem.
// --------

$ git shortlog  1c97e542..07222d0   --no-merges
Andras Lasso (1):
      BUG: Fixed issue of writing Python errors to stdout instead of stderr

Andreas Fetzer (1):
      Remove empty build command and set install command empty. Thus is cannot happen     that the dcmtk libraries are installed to some unintended location.     Setting dcmtk_dir to dcmtk-build directory

Christian Askeland (4):
      fixed crashing line in ctkFileDialog for Qt5.
      improved in-code description of previous ctkFileDialog crash fix
      Fixed broken doxygen generation for Plugin Framework
      set doxygen variable ensuring macro definitions of type MY_FOO(arg) dont mess up doc generation

Jean-Christophe Fillion-Robin (7):
      Update ExternalProjectDependency based on commontk/Artichoke@388fc33
      Update ctkMacroBuildQtPlugin to fix CMake warning related to CMP0026
      Fix generator expression introduced in previous commit d779b43
      Check bitness using void ptr size instead of system processor name
      Add PythonQt decorator for ctkErrorLogContext
      Update PythonQt decorator of ctkErrorLogContext to include set/get for file
      vtkLightBoxRendererManager - Fix warning related to missing active camera

Sascha Zelzer (1):
      Removed custom QT5_INSTALL_PREFIX variable.