Attempt to support different collections of Unsplash

Summary:
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}

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22494