Add support for compressed images outside of PDF files

This works for anything QImage supports, although only PNG is registered
so far, as that's practically the only format in use for barcodes.

This is mainly needed for porting KItinerary Workbench for now (which did
this handle this case manually so far), but we'll need that for supporting
images in HTML emails anyway.
3 jobs for master in 13 minutes and 2 seconds (queued for 36 minutes and 7 seconds)
Status Name Job ID Coverage
  Build
passed build_clazy_clang_tidy #98574

00:09:33

passed linux kf5-qt5 qt5.15 #98573

00:02:56

72.0%
 
  Test
passed reuse #98575

00:00:32