For some applications it is useful or even critical that they are started
only once by any user. Future attempts to start the application should
activate any already running instance, and possibly perform requested
actions, e.g. loading a file, in that instance.

The QtSingleApplication class provides an interface to detect a running
instance, and to send command strings to that instance.
For console (non-GUI) applications, the QtSingleCoreApplication variant is provided, which avoids dependency on QtGui.