Updated mixer

- Tidied up the code a little and grouped mixer types in model.js.
- Removed second motor from all fixed wing mixers that aren't differential thrust. A bug was discovered that if an FC only has a single motor output, and 2 motors are set in the mixer, the motors won't work.
- Added mixer for V-Tail with differential thrust
- Enhanced the new servo button. Clicking it now selects the next available servo number, rather than just 0.

js/servoMixerRuleCollection.js

@@ -99,5 +99,20 @@ let ServoMixerRuleCollection = function () {
         });
     }
 
+    self.getNextUnusedIndex = function() {
+        let nextTarget = 0;
+
+        for (let ruleIndex in data) {
+            if (data.hasOwnProperty(ruleIndex)) {
+                let target = data[ruleIndex].getTarget();
+                if (target > nextTarget) {
+                    nextTarget = target;
+                }
+            }
+        }
+
+        return nextTarget+1;
+    }
+
     return self;
 };