FindMagnum.cmake: allow user to override plugin directories.

Useful when deploying on Windows.
Renamed Mesh variables.

Use leading underscore to hint that they are private variables.
Doc++
Math: hardened checks for proper size of primitive types.
Trade: fixed wrong indentation.
Trade: don't forget to initialize variable.

It's just test, but it might fail when least expected.
Add performance recommendations for Intel Graphics on Linux.
Bootstrap Audio library.

Will require OpenAL for now, although I don't like the state in which AL
currently is. At all. Other alternatives (later) might be FMOD or Miles.
Added Doxygen aliases for pointing to OpenAL documentation.

Not found any online manpages however, so just placeholder.
Audio: verify OpenAL types.
Audio: added Renderer class.
Audio: added Buffer class.
Audio: added Source class.
Audio: added AbstractImporter plugin interface.
Audio: added Context class.