ENH: Enable testing infrastructure independently of BUILD_TESTING value

This is particularly useful when extensions build along Slicer using
-DSlicer_EXTENSION_SOURCE_DIRS want to leverage the testing infrastructure
without the overhead of running all Slicer tests.

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23837 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Allow displayable manager to be introspected using python

This is particularly useful when debugging issues.

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23838 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: More explicit debug message in displayable manager group

The class name of the registered displayable manager is displayed.

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23839 3bd1e089-480b-0410-8dfb-8563597acbee
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,...>.

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23840 3bd1e089-480b-0410-8dfb-8563597acbee