Introducing documentation with Sphinx

This PR introduces the generation of documentation based on
this excellent blog post: https://devblogs.microsoft.com/cppblog/clear-functional-c-documentation-with-sphinx-breathe-doxygen-cmake/

It combines the tools Sphinx, Doxygen and Breathe to generate good
looking HTML documentation conveniently which can be hosted easily.

The helper scripts were unified and there is now one helper.py script
which can be used to create, build and open both tests and documentation.
"./helper.py -h" can be used to get the different options.

This PR also contains some smaller fixes which were necessary for the docs
to build

docs/api/event.rst

@@ -0,0 +1,6 @@
+.. _eventapi:
+
+Event API
+============
+
+.. doxygenfile:: Event.h