Quit Fix

Uncommenting the GTK delete_event code appears to fix https://github.com/adobe/brackets/issues/4611

I tested on Arch Linux 64-bit, using the latest brackets-shell and brackets versions. 
according to the gnome docs, `gtk_widget_get_toplevel()` returns the topmost widget in the hierarchy so calling `gtk_widget_is_toplevel()` should always return true or am I not understanding how this works? Seems that we should just call `gtk_widget_get_toplevel()` and emit the `delete_event` on whatever widget that is returned.