GTK UI: On macOS cmd-q should quit the application

[phrohdoh]

See https://mail.gnome.org/archives/gtk-osx-users-list/2011-April/msg00036.html for some details.

[phrohdoh]

The quit event includes cmd-q.

https://github.com/GuillaumeGomez/process-viewer/blob/f4287f2803b4af558d667615f5778ab06c8e6b8e/src/process_viewer.rs#L308