Simplify checking whether an entity type is revisionable

I'd say we should mark the relevant entity types as revisionable :)

That is:

• custom_block
• node
• entity_test_rev
• entity_test_mulrev

I still think we should be using an explicit entity key instead of relying on the revision key, to make revisionability more easily alterable, anyway let's discuss this in a follow-up...

d8_entity_revisionable.patch no longer applies.

error: core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php: No such file or directory

Re-roll.

Yeah, I'm fine with this for now. @plach wants to make the entity key information non-mandatory and provide sensible defaults for them, in which case this has to become a separate property, but since this will centralize the check it should not be a problem to change the implementation at that point.

Also once we have it we can move isRevisionable() along with isTranslatable() onto ContentEntityTypeInterface

9: drupal_2224549_9.patch queued for re-testing.

- Fix unit test
- Switch from (bool) getKey() to hasKey() in isRevisionable()

Also providing a separate patch and interface for moving this method to ContentEntityTypeInterface. I personally think that's the better patch. It's certainly more correct and we will want to do that at some point, but maybe people will want to all appropriate methods at once. I.e. isTranslatable() should be on ContentEntityTypeInterface as well.

This should fix the test failures for the second patch in #16.

+1 on #19

Committed/pushed to 8.x, thanks!