#968 PLCgen: more explicit operator bindings, and fixed operator priorities for 'xor' and binary '-'

• Best to review per commit.
• End-user visible changes:
  • Fixed PLCgen operator priority for binary -.
• Other changes:
  • Operator priority and associativity is now configurable per target.
  • PLCgen: define expression bindings per operator.
    • The categories were confusingly named after one of the operators.
    • Make it explicit for each operator what the expression binding is.
  • Fixed PLCgen operator priority for XOR.

Closes #968 (closed)