Switch all digiKam bundles to last 5.70 which now support new Gimp XCF file format.

See KF5 release announcement : https://kde.org/announcements/kde-frameworks-5.70.0.php
"KImageFormats: Add support for modern Gimp images/XCF files"
This will permit to digiKam and Showfoto to show XCF thumbnail and load preview/full image (read only)
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