Add some standard EntryType metadata

[elia]

There are some features that are common among entry-types, among those:

• The ability to hide, publish, and preview entries, this means also having multiple entries for a single slug, all previewable, but with a single one (at most) published
• The ability to mark the entry type as a singleton (e.g. the home page), this will limit the entries to a single default slug
• The ability to have other mechanisms that will determine which entry should be presented given some characteristics such as geolocation etc. (e.g. having a metadata that store a list of countries for which the entry should be presented and fall-back to a default one for all others)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It might be closed if no further activity occurs. Thank you for your contributions.