Improve Doxygen support

Except Graphviz's `dot` Doxygen may use few other utilities like
`mscgen` (Message Sequence Chart) and `dia` (Diagram Editor).

Now this module allows to manage Doxygen settings from `CMakeLists.txt`
and forget about `Doxyfile`s. Also it provides a helper function
to add a target to generate documentation: `doxygen_add_docs`.

Implement code review notes:

- Introduce `COMPONENTS` to find: `dot`, `mscgen` and `dia`;
- Deprecate variables `DOXYGEN_SKIP_DOT`, `DOXYGEN_EXECUTABLE`,
  `DOXYGEN_DOT_EXECUTABLE`, `DOXYGEN_DOT_FOUND` in favour of
  `doxygen_add_docs ` usage instead;
- Properly handle paths to found tools in Windows;
- Prevent adding a custom target if Doxygen was not really found;
- Introduce exported (executable) targets for found components.

Co-Author: Craig Scott <craig.scott@crascit.com>

Help/release/dev/doxygen-improvements.rst

@@ -0,0 +1,10 @@
+doxygen-improvements
+--------------------
+
+* The :module:`FindDoxygen` module learned to control Doxygen behavior using
+  CMake variables and generate documentation via the newly added
+  :command:`doxygen_add_docs` function. The Doxygen input file (``Doxyfile``)
+  is automatically generated and doxygen is run as part of a custom target.
+  A number of doxygen-related variables have been deprecated. Additional
+  components can be specified to find optional tools: ``dot``, ``mscgen``
+  and ``dia``.