Axes: register as default
plotter.html.html_area: info -> alt
plotter.html: allow custom popup alt

    - either callable function that will take (x, y) pixel coordinates,
      or
    - iterable with one element per data point that will be directly
      copied
plotter.html: renamed tooltip class gwpy-tooltip
plotter.html: document popup kwarg
plotter.html.info_box: remove unused function