Retrieving discart

Refactored the code for retrieving artwork in state.cpp to allow for specifying an artwork type (along with 5 other variables needed for all the paths/data!). Added handling code in httpserver to return both file and embedded artwork.

I did not alter the ajquery theme at all, but I did update the config & state.json, plus I provided some sample fallback discart so that users can see how it all is supposed to work.

Scenarios tested for both discart and albumart since that code changed a little as well:

• Nothing playing

• No artwork found

• artwork found

• embedded only

• prefer embedded over file

• file over albumart_limit_size and no embedded

• file over albumart_limit_size falling back to embedded

• embedded over albumart_limit_size