Use Ctrl-Q for quitting the program on all platforms

Ctrl-Q is a more intuitive way of closing programs on Linux and on Mac-OS. Ctrl-W (which is the keybinding associated with QKeySequence.StandardKey.Close) is usually used for closing individual windows. Since it seems that, in general, there is no keybindings associated with quitting the application on Windows, then there would not be harmful to impose Ctrl-Q on all platforms.