Handled performance issues and bugs noticed during testing of refs #319 / refs #322

* discussion: https://dev.vatsim-germany.org/boards/22/topics/2027?r=2040#message-2040
* fixed bug with ATC station component, wrong signals for booked stations
* booked stations loading to frequently (for each minor change such as online), changed to timestamp based concept
* update booked stations with receiving ATIS/voiceroom to online
* CDigestSignal class: new class and methods for collecting signals, avoiding too many signals - one of the cures for the performance issues
* fixed bug found during testing, missing start for timers when connecting to network

src/blackgui/components/timerbasedcomponent.h

@@ -13,6 +13,7 @@
 #define BLACKGUI_TIMERBASEDCOMPONENT_H
 
 #include <QTimer>
+#include <QDateTime>
 
 namespace BlackGui
 {
@@ -28,6 +29,13 @@ namespace BlackGui
             //! Destructor
             ~CTimerBasedComponent();
 
+            //! Date/time of 1/1/1970, used to init timestamp values as "outdated"
+            static const QDateTime &epoch()
+            {
+                static const QDateTime e = QDateTime::fromMSecsSinceEpoch(0);
+                return e;
+            }
+
         public slots:
             //! Update time, time < 100 stops updates
             void setUpdateInterval(int milliSeconds);