Do not store temporary directories in the Dangerzone's config directory.
There are two reasons for that:
1. They are ephemeral, and they need a temporary place to be stored,
preferably RAM-backed.
2. We need to set them while running our CI tests.
- display_banner() was only displayed in CLI mode so it makes sense
for it to be in the CLI.
- get_version(), was mvoed to util since it is a static function
that is needed in multiple parts of the application.
static methods that are used application-wide should belong to
the utilities python file.
inspired by @gmarmstrong's PR #166 on refactoring global_common
methods to be static and have a dzutil.py
