Automatically export cpp.frameworks like for libraries

This applies to Apple platforms.
Currently a StaticLibrary that links against a framework using cpp.frameworks: [...] must also export that dependency for other targets that depend on it using Export. Otherwise the framework is not linked into the dependent target.

Consider the following qbs file:

  StaticLibrary {
    name: "some-library"
    files: ["source.m"]
    Depends { name: "cpp" }
    cpp.frameworks: ["Foundation"]
  }

  CppApplication {
    name: "some-app"
    files: ["another-source.c"]
    Depends { name: "some-library" }
  }

At build time, the linker will complain about symbols from the Foundation framework not being available. Changing the StaticLibrary above to:

  StaticLibrary {
    name: "some-library"
    files: ["source.m"]
    Depends { name: "cpp" }
    cpp.frameworks: ["Foundation"]
    Export {
      Depends { name: "cpp" }
      cpp.frameworks: ["Foundation"]
    }
  }

is required for the dependent application to link properly.

For convenience, the frameworks used by a target (library) should automatically be exported.