Graphviz: added test suite, fixes, enhancements

* Added a fairly comprehensive test suite
* Separated the graph traversal logic from the Graphviz generation
  code by introducing a new class, cmLinkItemsGraphVisitor{.h,cxx}
* Made the graph traversal logic less ad-hoc by using existing
  methods in the GlobalGenerator; this fixed a few bugs
* Added support for new target types: custom targets, object
  and unknown libraries
* Improved support for ALIAS libraries by showing the alias(es)
  in the graph
* Introduced new flags to control those new libraries (consistent
  with existing flags)
* Updated the documentation
* Removed useless setting to set graph type in dot file
* Improved the node/edge shapes (nicer, more consistent)
* Added a legend to the graph
* Some refactoring and cleanup of the Graphviz generation code
* Added test and fix for issue 19746