modules: removed obsolete code from UseCorrade.cmake.
modules: display resource files in IDE project view.

Currently the corrade_add_resource() macro was opaque, i.e. it didn't
add anything to project view (in QtCreator, e.g.). Now it adds all
gathered files to make users' lives easier.
modules: display also plugin metadata files in IDEs project view.
CMake: display also all header files in project view of IDEs.

Finally makes the development with QtCreator usable.