Add support for the new VTX settings API

When a controllable VTX is configured, a new section appears in
the configuration tab which allows the user to set the band, channel
power and "low power on disarm" option.

js/fc.js

@@ -39,6 +39,7 @@ var CONFIG,
     RX_CONFIG,
     FAILSAFE_CONFIG,
     RXFAIL_CONFIG,
+    VTX_CONFIG,
     ADVANCED_CONFIG,
     INAV_PID_CONFIG,
     PID_ADVANCED,
@@ -296,6 +297,15 @@ var FC = {
             capacity_unit: 0
         };
 
+        VTX_CONFIG = {
+            device_type: VTXDEV_UNKNOWN,
+            band: 0,
+            channel: 1,
+            power: 0,
+            pitmode: 0,
+            low_power_disarm: 0,
+        };
+
         ADVANCED_CONFIG = {
             gyroSyncDenominator: null,
             pidProcessDenom: null,