Make supportsManualControl a plugin option

src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.cc

@@ -57,3 +57,8 @@ bool ArduSubFirmwarePlugin::supportsThrottleModeCenterZero(void)
 {
     return false;
 }
+
+bool ArduSubFirmwarePlugin::supportsManualControl(void)
+{
+    return true;
+}

src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.h

@@ -71,6 +71,7 @@ public:
 
     bool supportsThrottleModeCenterZero(void);
 
+    bool supportsManualControl(void);
 };
 
 #endif

src/FirmwarePlugin/FirmwarePlugin.cc

@@ -89,6 +89,11 @@ bool FirmwarePlugin::supportsThrottleModeCenterZero(void)
     return true;
 }
 
+bool FirmwarePlugin::supportsManualControl(void)
+{
+    return false;
+}
+
 bool FirmwarePlugin::adjustIncomingMavlinkMessage(Vehicle* vehicle, mavlink_message_t* message)
 {
     Q_UNUSED(vehicle);

src/FirmwarePlugin/FirmwarePlugin.h

@@ -133,6 +133,10 @@ public:
     /// throttle.
     virtual bool supportsThrottleModeCenterZero(void);
 
+    /// Returns true if the firmware supports the use of the MAVlink "MANUAL_CONTROL" message.
+    /// By default, this returns false unless overridden in the firmware plugin.
+    virtual bool supportsManualControl(void);
+
     /// Called before any mavlink message is processed by Vehicle such that the firmwre plugin
     /// can adjust any message characteristics. This is handy to adjust or differences in mavlink
     /// spec implementations such that the base code can remain mavlink generic.

src/FirmwarePlugin/PX4/PX4FirmwarePlugin.cc

@@ -170,6 +170,11 @@ int PX4FirmwarePlugin::manualControlReservedButtonCount(void)
     return 0;   // 0 buttons reserved for rc switch simulation
 }
 
+bool PX4FirmwarePlugin::supportsManualControl(void)
+{
+    return true;
+}
+
 bool PX4FirmwarePlugin::isCapable(const Vehicle *vehicle, FirmwareCapabilities capabilities)
 {
     if(vehicle->multiRotor()) {

src/FirmwarePlugin/PX4/PX4FirmwarePlugin.h

@@ -44,6 +44,7 @@ public:
     void        guidedModeChangeAltitude        (Vehicle* vehicle, double altitudeRel) final;
     bool        isGuidedMode                    (const Vehicle* vehicle) const;
     int         manualControlReservedButtonCount(void) final;
+    bool        supportsManualControl           (void) final;
     void        initializeVehicle               (Vehicle* vehicle) final;
     bool        sendHomePositionToVehicle       (void) final;
     void        addMetaDataToFact               (QObject* parameterMetaData, Fact* fact, MAV_TYPE vehicleType) final;

src/Vehicle/Vehicle.cc

@@ -1517,15 +1517,7 @@ bool Vehicle::vtol(void) const
 
 bool Vehicle::supportsManualControl(void) const
 {
-    // PX4 Firmware supports manual control message
-    if ( px4Firmware() ) {
-        return true;
-    }
-    // ArduSub supports manual control message (identified by APM + Submarine type)
-    if ( apmFirmware() && vehicleType() == MAV_TYPE_SUBMARINE ) {
-        return true;
-    }
-    return false;
+    return _firmwarePlugin->supportsManualControl();
 }
 
 bool Vehicle::supportsThrottleModeCenterZero(void) const