Use "Scaled and Cropped" as the default wallpaper image fill mode

Summary: The current default setting is "Scaled", which results in images that don't match the screen's aspect ratio being compressed along the X or Y axis which almost never looks good. Cropping the image instead of deforming it is a much better default setting.

Test Plan:

• Create new user account
• Right-click on desktop > Configure Desktop
• See that "Scaled and Cropped" is the default setting for the Image and Slideshow plugins

Reviewers: VDG, Plasma, ndavis

Reviewed By: VDG, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18004