Release v4.0.0

What's changed

This release carries major code overhaul and refactor. Main change is replacing 7z with unsquashfs. Hopefully this will solve many issues on older systems that does not have 7z installed or have older versions that doe snot support zstd.

Main highlights:

• dropped 7z dependency in favor of native unsquashfs that is bundled by default the same way as dwarfsextract. Still, appimage-thumbnailer can be built without these binaries.
• removed dwarfs, keeping only dwarfsextract. It was required only for file listings, but we assume that all AppImages should have .DirIcon in root folder. Note: case sensitive
• improved debuging info to be more pronounced and verbal

Full Changelog: v3.0.2...v4.0.0