Show combined FLARM traffic from all available devices

The motivation is to support traffic streams from both FLARM and
OGN (Open Glider Network) devices and display it on a single "FLARM Radar"
screen.

OGN devices can output position of nearby aircraft using FLARM protocol
(i.e. using $PFLAA sentences), so that XCSoar or other FLARM displays
could recognize and display the traffic data. If an aircraft is equipped
with both FLARM and OGN device, and both transmit data to XCSoar, XCSoar
sees both devices as "FLARM", but displays traffic only from the first
device.

This patch fixes the problem by combining traffic lists from both devices.
If aircraft with the same ID appears on more than one device, the data
from only the first one will be used.

NEWS.txt

@@ -13,6 +13,7 @@ Version 7.0 - not yet released
   - FLARM/OGN - make it possible to set/download registered device database
   - device manager: show flag if device provides data from
       environmental sensors (temperature, humidity)
+  - combine traffic from all FLARM devices (support both FLARM and OGN devices on board)
 * weather
   - merge all weather data in one dialog
   - allow showing both terrain and RASP

src/FLARM/List.hpp

@@ -65,8 +65,12 @@ struct TrafficList {
    * this one.
    */
   void Complement(const TrafficList &add) {
-    if (IsEmpty() && !add.IsEmpty())
-      *this = add;
+    // Add unique traffic from 'add' list
+    for (auto &traffic : add.list) {
+      if (FindTraffic(traffic.id) == NULL) {
+        list.append(traffic);
+      }
+    }
   }
 
   void Expire(double clock) {