Upstream AppImage patches for squashfuse and squashfs-tools

We should try to send the AppImage patches to https://github.com/vasi/squashfuse and https://github.com/plougher/squashfs-tools to be able to drop these patches and not have to build and ship our own libraries. This would also make packaging for distros easier.

Patches to send:
- [x] https://github.com/AppImage/AppImageKit/blob/appimagetool/master/src/mksquashfs-mkfs-fixed-timestamp.patch
- [ ] complex squashfuse patches: https://github.com/AppImage/AppImageKit/blob/appimagetool/master/src/patch-squashfuse.sh.in

The squashfuse patches are pretty complicated, and we'll see how this will turn out.