LVAR for MX170B tuning selection

[fizzboy]

@Black-Square any chance we can get an LVAR that reads the current MX170B tuning selector? I hacked it in so that I can get my external radio panel to reliably reflect the current NAV/COM selection and figured other folks might find it helpful.

case "Button_NavComSelect":
  this.tuningSelection = !this.tuningSelection;
  SimVar.SetSimVarValue("L:var_Mx170bTuningSelection_"+this.radioIndex+"_readonly", "Bool", this.tuningSelection)
break;