AppImage recipes are the legacy method for building AppImages, whereas
linuxdeploy is meant to replace them. It takes care of finding and
deploying all needed libraries and Qt plugins automagically, but it's
still under heavy development, so some manual work using appimagetool
and a few fixes are required to make it work for KeePassXC.
This patch moves building of AppImages directly into the release-tool,
both as a separate module (release-tool appimage) as well as a flag
(--appimage) for the build module. The release and CI Dockerfiles were
updated accordingly to support the new build process. The release
Dockerfile also received a Qt update to version 5.10.1.
In theory, it is now possible to use release-tool appsign for embedding
PGP signatures into AppImages, but it fails in practice due to ELF
header size limitations.
