We should remove the hack in ConfiguredActionbarSectionDefinition#setAvailability, always setting section availability for multiple selection to true.

In effect, we cannot simply remove it because most—if not all—content-apps are misconfigured.
The multiple property is never configured, for sections intended for multi-selection.
So multi-selection would result in an empty action bar.

Besides, as quirky as this is, we don't want to change behavior that could potentially affect apps for existing customers.

1. We fix the UI def, extend it with a deprecated one (with the hack), and register it as default type-mapping.
   • This way, the hack moves to a deprecated class, and we can eventually phase it out at some point in the future.
2. We don't try to address potential usages in apps, not much worth the pain. Instead, action bar config may be re-evaluated (different ticket).

See https://wiki.magnolia-cms.com/display/WIKI/Options+for+solving+multiple+actionbar+availability for details.