fetchart: Option to download all defined sources and choose the biggest one

### Use case

I have a `minwidth` of 1024 for `fetchart` plugin. `source` priority is filesystem.

My `cover.jpg` image in filesystem is 500x500. Because of that, it tries to get a better picture using the `fetchart` plugin.

Problem is: the fetched image is 300x300. In this situation, since the fetched image is `worse` than my filesystem image, I would prefer to use it instead of the downloaded one.

### Solution

I would like to have a better approach, but don't know exactly what to do:

- Create a new config, `download_all` that tries to get the image from all sources, and chooses the best one? This is a completely new development of the fetchart feature;
- Create a new config, `fallback_worse_fetched_ratio`, that uses the filesystem `cover.jpg` instead of the fetched image if the downloaded resource has a worse ratio.

### Alternatives

Today is a manual approach.