Do not modify ellipsize property within a size request function

Instead, we can use the natural size request which is equivalent to the non-ellipsized size.
See: https://developer.gnome.org/gtk3/stable/GtkLabel.html#label-text-layout

https://bugzilla.gnome.org/show_bug.cgi?id=697198
window: remove custom get_preferred_width/height implementation

We already take care of the default size when we create the window, and
these only break assumptions of the default GtkWindow's handlers.