Scale up folder icon before creating preview overlays

Summary:
If a custom folder icon is smaller than the requested preview size
KIO::PreviewJob returns a pixmap of only that size. E.g. dolphin
scales this up to its current thumbnail size which results in ugly
overlay icons for very small folder icons. If the folder icon is
scaled up before creating the overlay icons the overall preview
looks much better.

CCBUG: 315983

Test Plan:

1. Set a small custom icon (e.g. 32 px) for a folder containing images
2. Start dolphin with Preview enabled in the parent folder
3. Zoom thumbnails to e.g. 128 pixels overlay
4. The overlayed preview icons should not be blurry

Before:	After:

Reviewers: Frameworks, rkflx

Reviewed By: rkflx

Subscribers: ngraham, rkflx

Differential Revision: https://phabricator.kde.org/D12734