Add sensor button, fix sensor code.

lib/sensors.ks

@@ -24,5 +24,31 @@ function ReadSensor {
   if not sList:HasKey(s) {
     return -1. // TODO: is -1 an impossible result for all sensors? I suspect not...
   }
-  return s:DISPLAY:TOSCALAR.
+
+  if not sList[s]:ACTIVE {
+    sList[s]:Toggle().
+  }
+
+  // UGH, this appears to be the only way to do this.
+  local ret is -1.
+  if s = "PRES" {
+    set ret to SHIP:SENSORS:PRES.
+  }
+  if s = "TEMP" {
+    set ret to SHIP:SENSORS:TEMP.
+  }
+  if s = "ACC" {
+    set ret to SHIP:SENSORS:ACC:MAG.
+  }
+  if s = "GRAV" {
+    set ret to SHIP:SENSORS:GRAV.
+  }
+  if s = "LIGHT" {
+    set ret to SHIP:SENSORS:LIGHT.
+  }
+
+  // turn off the sensor when we're done with it
+  sList[s]:Toggle().
+
+  return ret.
 }