Switch to independend versioning

Created by: elbenfreund

Currently, the shell extension does not have a proper versioning scheme. The repository uses tags to refer to compatible gnome versions instead whilst the extension metadata.json uses a simple build counter. In order to be more explicit we should switch to a proper (semantic) versioning scheme. Specifying gnome shell (in-)compatibilities is up to the documentation and the extensions metadata.json.

An added bonus will be proper usage of project milestones.