Fix some more warnings in vtkDispatcher.

The "rule of three" states that a class that declares a destructor
should also declare an assignment operator and copy constructor.
In fact, C++11 deprecates their implicit generation if a destructor
exists.  Therefore, they should be explicitly declared.

I've also removed the private default constructor that I added to
FunctorHandler in a previous commit, it served no purpose.

Change-Id: I4797fe9864c741d3c5e30a11b8ab186ab56b4779
Merge topic 'dispatcher-warnings' into master

0a68fe20 Fix some more warnings in vtkDispatcher.