1. I used Unsplash Source API (unlimited, public) instead of parsing HTML. It should be faster and more reliable. (Parsing HTML may break anytime they update their website...)
2. Create `UnsplashBaseProvider`, which has `collectionId` in constructor. Extend it in children classes to support different collections. Now support all wallpaper collections.
{F7034344}
{F7037556}
{F7037557}
{F7037558}
{F7037563}
{F7037567}
{F7037570}