Allow clients to register custom artwork resizing specifications

Controller applications fetch artwork from the server in various sizes.
Unless they happen to match one of the pre-cached sizes, loading an
album list can be very slow. This new feature allows such applications
to register a resizing specification which would then pre-cached during
a media scan. This way loading album lists would hit the cache instead
of requiring each artwork to be rendered at load time.

Developers - please send a "artworkspec add 123x123_o My%20Controller"
command to the server when you connect. See CLI documentation for
detils.