Add Simon Stalenhag POTD Provider

Hello! 🙂 Introducing the support of the Simon Stalenhag's website as Picture-Of-The-Day provider.

From Wikipedia:

Simon Stålenhag is a Swedish artist, musician, and designer specialising in retro-futuristic digital images focused on nostalgic Swedish countryside alternate history environments.

Talking about contacts with the artist, I guess I could share this MR with Simon via his twitter. He seems very open to the fans.

Talking about technical details: since the website have no API, I'm proxying metadata about existing artist's works via public GitHub repo. It is also possible to do it directly via regexp on the qml or cpp side, just wanted to make the code cleaner.

Here's my similar project for the Sailfish OS (also shared with the artist via twitter few years ago): https://openrepos.net/content/aa13q/simon-stalenhag-imagitron-art-gallery

Feel free to criticize or close. Thank you! :)

Here's the testing screenshots: