Support for HellaNZB's fillserver option

Explanation by Jacques Perrier:

"The fillserver option is useful to declare secondary server(s) to only be used if the content to download is not present on the primary (nonfill) server(s).
It is usefull for example if you a have a short retention unlimited account and a longer retention limited one.

nonfill server(s) : fillserver = 0 (default option value)
fill server(s) : fillserver > 0, servers with fillserver = 1 being used before fillserver = 2, etc..."

There are two possible ways to implement this blueprint.

== Changing the server list ==
Adding another spin-button to the server edit dialog doesn't sound like a viable option and isn't that user-friendly imho. What about adding drag&drop support to the server list? The server at the top would get the lowest fillserver value (0 so to say) while the ones following would get a higher value. It might be necessary to display an explanatory hint as soon as there are more than one server.

== Changing the server dialog ==
Make the server dialog depend on the number of servers that have already been specified.

- If the user is adding the first server or editing the only existing server, don't display anything related to the fillserver option.
- If the user is adding a second server or editing one of two existing servers, display a checkbox that let's the user specify if the server should be a fillserver or not. Don't let the user mark both server as fillservers (somehow ;-)).
- If the user is adding a third, fourth, etc. server or editing one of three or more servers, display a spin button (like the connection option) with a one-line explanation below that that 0 is the highest priority etc.

Users who only use one server won't be affected by the change at all.