Refs #10843. Adds IsExiting() call to SignalHandling.

This allows programs to exit on a signal when they are not in qApp->exec().
They just need to call QCoreApplication::processEvents() and then check
this return value and exit if appropriate.

mythtv/libs/libmythbase/signalhandling.h

@@ -23,6 +23,8 @@ class MBASE_PUBLIC SignalHandler: public QObject
 
     void AddHandler(int signal, void (*handler)(void));
 
+    static bool IsExiting(void) { return s_exit_program; }
+
     // Unix signal handler.
     static void signalHandler(int signum);