Increase compatible Oxygen version for the addon and update framework resources

[AlexJitianu]

Right now the add-on is compatible with Oxygen 18.1 or newer which mean that the framework resources, like CSSs for the author mode, author actions, are those from Oxygen 18.1. As long as we keep this compatibility, we can't commit any new resources. Perhaps it's time to increase this version as it's been 7 years since Oxygen 18 was released. If we agree on a version increase, I will send a pull request with the updated resources.

Last time when we moved from version 16 to 18.1 it was because the edit facsimile actions which were added in version 18.1.