Improve release.sh

`release.sh` kind of got in my way while I was working on the i18n stuff (#2642), so I'm proposing some changes in this PR:

- removal of `version.txt`, that information is usually available elsewhere (git tags, `PKG-INFO` file)
- semi-automated handling of `branch.sql` (not full automation like in #2360)
- automated version numbers (with the format switched from "xx.x.x" to "xxxx")