add example gui panel for any plugin-specific config options users need to configure

Most plugin users probably don't want to write wx dialogs from scratch, but could hang with modifying one if they needed to set a parameter or two. See pyme-omero [install_plugin.py](https://github.com/python-microscopy/pyme-omero/blob/master/pyme_omero/install_plugin.py) and [ui dialog](https://github.com/python-microscopy/pyme-omero/blob/master/pyme_omero/ui/credentials.py) for an example. 