Snap: Fix file chooser icons. Add jpeg 2000, gif, raw, TIFF, HEIF
This MR fixes a bunch of issues:
- file chooser has no icons: https://bugs.kde.org/show_bug.cgi?id=424447
- Support for jpeg 2000, gif, raw, TIFF and HEIF images
With this MR, the only disabled optional features according to cmake are Poppler, SIP and PyQt. Adding those will require some more work. @rempt do you know what those features are used for? Are these important for Krita?