* 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
* some improved comments and information
* QMetaObject::invokeMethod in tool.cpp as thread safe invocation
* common base class for threaded readers
* removed event class, using QMetaObject::invoke instead for forcing calls in main event loop
* stop methods for readers, as used for graceful shutdown (preparing for thread safe destruction of objects)
* graceful shutdown for network context
* calls in tool now via inkoke for thread safety (only thread safe methods called directly)
