Backport [3/5]: Move toolkits inside their own namespace

[rapgenic]

Not much else to explain :)

[rapgenic]

I kinda agree with you, I do remember not liking it much too, but it helps readability here:

  // Register the graphics toolkits
  xeus_octave::tk::notebook::register_all(interpreter);
  xeus_octave::tk::plotly::register_all(interpreter);

Plus one of the next PR is about flattening the source tree, so I'd keep it like this if you're ok with it..