Fix compilation of tests against VTK6

This commit allows CTK to be compiler against VTK6 with testing enabled.
That said, there are some tests failing to pass. Issues #467, #468, #469
and #471 track the failure most likely related to VTK6.
Merge branch 'fix-test-compile-against-vtk6' into 3361-tweak-ctkVTKConnection

* fix-test-compile-against-vtk6:
  Fix compilation of tests against VTK6
Fix qDebug operator to handle case when VTKObject is null

In case the debug statement is used in the destructor, this commit
will prevent the application from crashing.
Remove unused signal "isBroke()"
Simplify handling of VTKObject reference using vtkWeakPointer

Instead of observing the DeleteEvent associated to the VTKObject and
set it to zero after the object is deleted, let's simply use a vtkWeakPointer.

Let's note by using a weak pointer, the connection is not explicitly
disconnected.
Do not explicitly disconnect slots in ctkVTKConnection dtor

Since (according to Qt documentation [1]) "all signals to and from the object
are automatically disconnected" there is no need to explicitly disconnect
them.

This commit will also fix already disconnected type of error (See [2]).
Indeed, if we consider the following object hierarchy:

  * B derive from A
  * with A defining the macro QVTK_OBJECT
  * B setting up connection (using for example ctkVTKConnect()) with slot
  defined in B

when object B will be destroyed, connection involving slots defined in B
will be removed. Then, when the base class A (aggregating QVTK connection
object) will be destroyed, the connections will attempt to disconnect
non existent connection.

[1] http://qt-project.org/doc/qt-4.8/qobject.html#dtor.QObject
[2] http://www.na-mic.org/Bug/view.php?id=3361