Fix problem for registrering parameter at runtime.

• Add a macro that enables to automatically register the parameter to QMETATYPE system when loading the library.

  // IN HEADER FILE
  DTK_DECLARE_PARAMETER(dtk::d_path);
  
  // IN CPP FILE
  DTK_DEFINE_PARAMETER(dtk::d_path, d_path);

  The second argument of the second macro is just a dummy name to prevent redeclararion of an internal dummy variable.