* 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* Using CORRADE_CXX_FLAGS instead of our own (heavily outdated) set.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.