Magic drag and drop understands H-boxes

[jvdwetering]

This makes some of the convienence features for rewriting Z- and X-spiders available to H-boxes.
In particular:

• double clicking a Hadamard edge turns it into an H-box, and vice versa.
• The magic drag and drop now also works for some actions involving H-boxes. In particular, dragging an X(0) onto an H-box 'explodes' it, A X(pi) gets fused into it, a Z(pi) also 'explodes' it (i.e. copies through).

I think I also want to implement that you can drag a Z- or X-spider onto a H-box representing a Hadamard in order to do color change.

See also #162 . This PR requires some new pushes onto the PyZX repo.