Fix Segfault von Ctrl-Q

Calling Gtk::Main::quit from an application with an active window causes a segfault. To reproduce, open pavucontrol and press Ctrl-Q.

Fixed by closing the window instead.

cf https://stackoverflow.com/questions/50342151/what-is-the-difference-between-gtkmain-and-gtkapplicationcreate

Fixes #93 (closed)