Profile, page layout, override and display interfaces: setters return static, revert() split, docblocks

Problem/Motivation

Child 3 of #3624396: Plan: Harden InstanceInterface, IslandInterface and DisplayBuildableInterface as public API before RC1, split in two: this one is the interfaces around the instance, child 3a the instance itself.

• ProfileInterface::setIslandConfiguration() and PageLayoutInterface::setSources() return nothing, where a Drupal setter returns the object.
• DisplayBuildableOverrideInterface::revert() does two jobs under one name: it empties the override field and saves the content entity at once, a permanent write, and it returns the sources of the default display. PublishableInterface::revert() on the instance has the same name and another meaning: it puts those sources in the draft, as an undoable step.
• DisplayBuilderEntityDisplayInterface::isDisplayBuilderEnabled() declares no return type.
• The cardinality docblocks say any number that is not strictly positive means unlimited. The code treats only a negative number as unlimited: a cardinality of 0 means always full.
• PublishableInterface still speaks of saving where it means publishing.

Proposed resolution

• setIslandConfiguration() and setSources() return static.
• DisplayBuildableOverrideInterface::revert() splits in two: getDefaultSources(): array, which only reads, and deleteOverride(): void, which empties the field and saves the entity. revert() is removed, with no deprecation: the API freezes at RC1. Instance::revert() uses the two new methods.
• isDisplayBuilderEnabled(): bool.
• The cardinality docblocks say what the code does: DisplayBuildableInterface::CARDINALITY_UNLIMITED, a negative number, means unlimited.
• PublishableInterface docblocks speak of publishing. HistoryInterface::setNewPresent() names its argument as Instance does.

Remaining tasks

None.

User interface changes

None.

API changes

• ProfileInterface::setIslandConfiguration() and PageLayoutInterface::setSources() return static, were void.
• New DisplayBuildableOverrideInterface::getDefaultSources() and deleteOverride(). revert() removed.
• DisplayBuilderEntityDisplayInterface::isDisplayBuilderEnabled() returns bool.

Data model changes

None.