EogWindow: Fix refcount and signal handler issues around GSettings handles

This fixes several refcount issues and not disconnected signal handlers for EogWindow's GSettings handles.

• Due to some leaked references the window itself or its EogThumbNav could be prevented from finalizing. This would keep the GSettings bindings alive, which might cause a crash if the binding triggers
• EogWindow uses standard signal handlers to connect GActions to GSettings properties. If the GSettings handle was not finalized together with the window, these were not disconnected and could still trigger. As the window and its actions were gone this would crash. Fixes #291 (closed).