FindMagnum: added also MAGNUM_WINDOWLESSAPPLICATION_* aliases.
Platform: added MAGNUM_WINDOWLESSAPPLICATION_MAIN() alias.

Treating windowless application differently to windowed ones, as they
don't share any API with them.
Platform: have special MAGNUM_*APPLICATION_MAIN() for each class.
Platform: added Application and WindowlessApplication aliases.

If only one *Application or Windowless*Application header is included,
the class is aliased to Application or WindowlessApplication to simplify
porting.
Link to namespace-related documentation from namespaces.
Updated FindCorrade.cmake from Corrade repository.
Added dummy documentation file for Platform namespace.

Will be written later, hopefully.
Documented also CMake side of platform portability.
Documented CMake usage and preprocessor variables.
Fixed typos (ahem).