extensionBase: Use UUID as fallback gettext domain

Nowadays we can set up translations automatically, but only when the domain is provided with the metadata.

This can be a problem, for example our extension templates use the automatic translation handling that we recommend, but as a result the code will throw an error unless gettext-domain is added to the metadata.

Binding a text domain is cheap enough to not care about unnecessary or duplicated calls, so add a final fallback to the UUID when no domain was provided as parameter or in the metadata.

There is already a precedent with the gnome-extensions pack command that falls back to the UUID when no explicit gettext domain was included in the metadata.