Subclass QApplication and move most code from main() there

Create a new subclass of the QApplication class and move most of the
code in the main function to the constructor of the new class. This
makes the code a bit more consistent in terms of object orientedness,
makes it a little simpler and allows extending the new Application class
in the future.
Update version number in MacOS bundle configuration file
Implement file associations on MacOS by reacting on QFileOpenEvent

Remove the deactivated MacOS dependent code for reacting on Cocoa events
for file associations from main.cpp

Try to implement the same functionality by reacting on QFileOpenEvent in
the Application class.