Auto-close viewer if all images in folder are deleted

[SumatraPeter]

When the last image in a folder is deleted, currently a blank page is displayed, while the titlebar displays some incorrect info.

Instead it would be nice if the viewer window was auto-closed when the very last supported image in a folder is deleted. If some people want the viewer to remain open with a blank screen (don't know why), "Auto-close window when last image is deleted" can be made an option.