Allow the string 'XOR' as a token for typed expressions

While ⊕ is used for the XOR pushbutton, most keyboard layouts cannot type ⊕ directly so allowing the 'XOR' string is useful when typing bitwise expressions. Using '^' would be more in line with '&' and '|' but that's taken by the power operator already.

This particular comparison is case-insensitive because a user is likely to type in lower case but the logical gate is properly written in upper case so a case-insensitive comparison seems to make the most sense