Add infrastructure for generating documentation of Lua API

[cschreib]

Currently, the Lua API of lxgui is undocumented. It follows closely the original WoW API, but:

• The WoW API has far more functions that lxgui does not (related to locking the interface to prevent cheating, or custom WoW-specific widgets, etc.)
• The WoW API isn't always super well documented (there are several websites and the quality varies)
• There are a number of differences between the functions in common between the two APIs

Acceptance criteria:

• [ ] The Lua API should be integrated in the Doxygen output if possible. This can use lua2dox. Not possible.
• [x] The Lua API should have its own separate section so it doesn't end up mixing up with the C++ API
• [x] The Lua API documentation is in the glue source next to the function, if at all possible.
• [x] There are a few examples of documented functions, to specify how the documentation should be written (what level of detail, conventions, etc.).

Once this is done, getting the API fully documented can be a background task.

[cschreib]

Started this with https://github.com/lunarmodules/LDoc in branch https://github.com/cschreib/lxgui/tree/lua-doc. It seems not possible to get Doxygen to reliably include Lua documentation, so I'm going for a separate output for C++ and Lua. It's a bit of a shame, but there should be a way to link the two together.

[cschreib]

The Lua API documentation is now available here https://cschreib.github.io/lxgui/lua/index.html