Updated FindCorrade.cmake from Corrade repository.
Thorough CMakeLists cleanup.

 * Explicitly installing header files instead of recursive glob (some
   header are internal, thus no need to install them).
 * Calling enable_testing() only in project root.
 * Using MAGNUM_LIBRARIES instead of specifying all dependencies
   explicitly.
 * Using CORRADE_CXX_FLAGS instead of our own (heavily outdated) set.
 * Removed FindMagnumPlugins.cmake, the module does essentially nothing.
 * Various reorganization and cleanup.