Dry run for 6.0.0

This pre-release is a release candidate for the next major version, in order to iron out the last kinks coming from the major changes (and to give packagers a chance to test it):

• rofimoji is now managed with poetry
• a large refactoring and splitting of the code base
• and finally, an example for a rofi theme

Please report any bugs you find! 🙂