Platform: removed unneeded include.
Platform: hide those ugly *ContextHandlers into Implementation namespace.

They are now not exposed to the user at all, as their API will be
probably changed and reworked in the future to allow GLX/EGL-specific
extension to be used. Also this looks scary in the documentation.
Platform: thoroughly documented building and usage with CMake.
Thoroughly documented also building and CMake usage of all other libs.
Updated Platform support documentation.

Now using generic aliases implicitly everywhere.