IO: Added "Super" keyboard modifiers (corresponding to Cmd on Mac and Windows key in theory although the later is hard to read) (#473)

NB: Value not used.

examples/sdl_opengl_example/imgui_impl_sdl.cpp

@@ -143,6 +143,7 @@ bool ImGui_ImplSdl_ProcessEvent(SDL_Event* event)
             io.KeyShift = ((SDL_GetModState() & KMOD_SHIFT) != 0);
             io.KeyCtrl = ((SDL_GetModState() & KMOD_CTRL) != 0);
             io.KeyAlt = ((SDL_GetModState() & KMOD_ALT) != 0);
+            io.KeySuper = ((SDL_GetModState() & KMOD_GUI) != 0);
             return true;
         }
     }