Add a new setting to toggle notification sounds on or off. Plays the
systems default instant messaging message sound (message-new-instant)
whenever a notification is shown if toggled on.
Fixes these kind of warnings:
```
(dino:<PID>): Gtk-CRITICAL **: gtk_window_set_transient_for: assertion 'parent == NULL || GTK_IS_WINDOW (parent)' failed
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
(dino:<PID>): GLib-GObject-WARNING **: invalid cast from 'GtkPaned' to 'GtkWindow'
```
Also centers these dialogs (settings, add chat, join conversation) on
top of their parent windows.
