// Go does not process the command line for flags (that is, it does not call flag.Parse()), nor does package ui add any of the underlying toolkit's supported command-line flags.
// AppQuit is pulsed when the user decides to quit the program if their operating system provides a facility for quitting an entire application, rather than merely close all windows (for instance, Mac OS X via the Dock icon).
// You should assign one of your Windows's Closing to this variable so the user choosing to quit the application is treated the same as closing that window.
// If you do not respond to this signal, nothing will happen; regardless of whether or not you respond to this signal, the application will not quit.
// Do not merely check this channel alone; it is not guaranteed to be pulsed on all systems or in all conditions.