Disable default NFT image download on GUI

[wintermute2]

I don't like the idea that anyone can send my wallet NFTs that include a url that my computer automatically fetches from a potentially malicious server. I'd feel much more comfortable if I could set a default for all NFTs to not automatically pull down the images onto my computer until I decide, based on the NFT information, that's its a good idea. My concern is not just the image content and potential malicious packages but also the ability to map out high value NFT ownership by IP address.