CLAP: Host GUI

[mfisher31]

      // clap_host_gui
      virtual bool implementsGui() const noexcept { return false; }
      virtual void guiResizeHintsChanged() noexcept {}
      virtual bool guiRequestResize(uint32_t width, uint32_t height) noexcept { return false; }
      virtual bool guiRequestShow() noexcept { return false; }
      virtual bool guiRequestHide() noexcept { return false; }
      virtual void guiClosed(bool wasDestroyed) noexcept {}