Two new switches have been added to the application preferences:
- Enable send button
- Use Enter to insert newline ('\n')
The latter cannot be active or sensitive if the former is not active.
Otherwise, users would not be able to send messages.
Thanks to horazont for suggesting a separate switch for the behaviour
of the Enter key.
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.
