Add gimp_color_panel_dialong_response() to GimpColorPanel, which
emits a response for the color panel's color dialog, if shown.

Add a "response" signal to GimpColorPanel, which is emitted upon
color dialog response.

In both cases, the response is a GimpColorDialogState, which should
be either GIMP_COLOR_DIALOG_OK or GIMP_COLOR_DIALOG_CANCEL, and not
an actual dialog response id.