Fixed loading of the wallpapers in the activity switcher

Summary:

• Created an asynchronous image provider to return wallpaper thumbnails
• Fixed the code that fetches the activity backgrounds from the config file
• Removed thumbnail cashing since we are relying on KIO now
• Removed the fun callback-based code from the QML/JavaScript side

BUG:360034

Reviewers: Plasma, davidedmundson, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2810