Fix and uniform the usage of Graph and Parent typedefs (#268)

- Rename Graph typedefs to GraphType in the implementation of graph
maps and MapExtender to prevent conflicts (especially using VS).
They are not public.
- Make Parent typedefs private in all classes.
- Replace Digraph with Graph in some places
(fix faulty renamings of the script).
- Use Graph and Digraph typedefs (more) consequently.

 INCLUDE(FindPackageHandleStandardArgs)

 FIND_PROGRAM(GHOSTSCRIPT_EXECUTABLE

   NAMES gs gswin32c

   PATHS "$ENV{ProgramFiles}/gs"

   PATH_SUFFIXES gs8.61/bin gs8.62/bin gs8.63/bin gs8.64/bin gs8.65/bin

   DOC "Ghostscript: PostScript and PDF language interpreter and previewer."

 )

 FIND_PACKAGE_HANDLE_STANDARD_ARGS(Ghostscript DEFAULT_MSG GHOSTSCRIPT_EXECUTABLE)