Add wallhaven.cc to the wallpaper provider

[trmdi]

Wallhaven.cc has quite a lot of nice wallpapers. What do you think about adding this wallpaper provider?

[morceaudebois]

So this is an interesting one! We use Unsplash as our main (and currently only) image provider for a few reasons:

• They have a (free) API, which allows us to easily integrate their data into Bonjourr. Their API is great because it provides lots of options and settings, like the average color of an image or the ability to provide images in specific resolutions.
• Artists on Unsplash upload their work themselves, which means that they're 100% willing to share them and want them to be used in projects like Bonjourr.
• They have a "collections" system, which allows us to make a selection of specific photos we want to integrate into Bonjourr. Bonjourr's default backgrounds are not just random image from Unsplash, but a carefully curated collection.

Unfortunately, Wallhaven in somewhat lacking in all of these departments. They do have an API, but it's a bit limited compared to Unsplash's. They don't seem to have a collections system apart from the favorites, and I don't know if you could access the favorites from their API.

But the biggest issue to me is the artist-crediting one. Although I agree that Wallhaven has lots of great wallpapers, most of them aren't uploaded by the original author, which means that they probably haven't agreed to having them on the site. Lots of images don't even have a source, and integrating them into Bonjourr wouldn't really be ethical from our standpoint as developers.

I wouldn't mind using them personally (as wallpapers on my computer or in my Bonjourr installation), and this is what the local files feature is for, but unfortunately that's about it.

We really appreciate your suggestion and pull request though. Thank you for that! 😊