Extract KSanePreviewImageBuilder from KSanePreviewThread

Summary:
This simplifies KSanePreviewThread and reduces differences
between KSanePreviewThread and KSaneScanThread.
The new class can be used later to implement building of
a preview image from KSaneScanThread.

Reviewers: sars

Reviewed By: sars

Differential Revision: https://phabricator.kde.org/D14993