Refactor i18n to not use global scope

[jorg-vr]

This pull request removes the I18n object from the global scope.
Instead the i18n object can be imported from i18n/i18n.

It also introduces the DodonaElement as a new meta webcomponent used to inherit from.
This component is a combination of the ShadowlessLitElement, the StateController and the i18nMixin.
A lot of custom components used at leased two of the three parts, so making a generic combined class made it easier to include all. I particularly introduced it in this pr because multiple web components used i18n without the i18nMixin which might result in untranslated elements on the page.

I also made some minor changes in the I18n class to fix typing issues:

• I renamed the accessor locale to loc as locale is defined as a function in Polyglot, which we extend.
• I removed the usage of super.warn, as this is a private function in Polyglot. I instead directly called the warning method they used directly.