Boolean variables not represented correctly visually in both edit and proof modes

[caldwellb]

From a discussion at the qpl hackathon, a spider with a boolean variable b should be represented bπ, however this is not currently the case. This is an issue with both pyzx and zxlive, as pyzx does not represent the boolean variable as bπ but just as b, but changing a local version of pyzx to return bπ when the variable b is a boolean did not cause zxlive to represent that variable correctly, so I believe there is also a bug in how zxlive sets variables to be booleans using the dropdown menu.

Relevant files in pyzx:
pyzx/symbolic.py