Updated FindCorrade.cmake from Corrade repository.
More flexible ability to install CMake modules.

FindMagnum.cmake is not installed by default to avoid the need for root
privileges when installing, enable it with WITH_FIND_MODULE CMake flag.
Always install Test/AbstractOpenGLTester.h.

It's counterintuitive when you have to enable `WITH_GL_TESTS` just to
install the header, moreover the dependent projects have no easy way to
detect the header presence, so they just fail on preprocessor error.
doc: updated Getting Started Guide to reflect changes in magnum-bootstrap.