#318 File: Allow online links for images

[gnosygnu]

As per this thread: https://sourceforge.net/p/xowa/discussion/defects/thread/8d5c4b4b/

XOWA shows images by either downloading them, or retrieving them from the image databases. However, HTTP server is tricky, since there isn't a way for the client to refresh the images. On the other hand, the Firefox Addon and the XOWA GUI allow the server to send messages to the clients.

One suggestion is to allow direct links to the online image. This would work well for HTTP Server, and can potentially be useful for Firefox Addon / XOWA GUI.