Unpublish/Delete nodes not included in feed

I've started with writing a kernel test for this feature. During writing the tests I found out that for the directory fetcher configuration schema is missing (same goes for the upload fetcher). I also added a new import() method to the Feed class with which you can import a feed in one go, which is in particular useful for the tests. Also noticed that $feed->startCronImport() results into a fatal error because the Feed class tries to call a method with the same name on a FeedImportHandler instance, but that class doesn't have that method. I should open separate issues for these findings, but as the end of my work day approaches, I note them here for the time being.

The test that is added by this patch should fail. All other tests should hopefully pass.

This adds an UI for configuring the action to take when items are no longer in the feed. It involves integration with the action plugin API! This means that actions like 'Delete content', 'Unpublish selected content' and 'Make selected content sticky' become available options. The list of actions is currently limited to those that are not configurable as these are harder to integrate in Feeds because they require additional UI and additional config.

The next step is to create methods to actually apply these action plugins.

For a part of the patch in #4, I had created a spin-off issue to decrease the complexity of this issue: #2941557: Add a Kernel test for DirectoryFetcher.. Most changes not directly focussing at the core of this issue where handled in that one. Only for calculating dependencies of the processor plugin an other spin-off issue may be worth it.

Since I committed #2941557: Add a Kernel test for DirectoryFetcher., the patch in #4 no longer applies, so here is a reroll that also makes use of the traits that were added in the other issue.

This adds the methods clean() and getImportedItemIds() to the EntityProcessorBase class. Test coverage is added for these two methods in the EntityProcessorBaseTest class.

The whole system of triggering the clean action is not yet implemented. This patch just adds a method to apply the clean action.

Coding standards + fixes in tests.

This patch adds the clean stage. Implementation is a bit experimental and currently works only when doing an import directly (programmatically). It doesn't work under normal conditions yet.

I had accidentally turned off UpdateNonExistentTest::testDeleteNonExistentItems().

Introducing the CleanState concept.

Removing implementation from interface.

Coding standards and test fixes.

Batch support.

Test and code style fixes, small refactoring in tests.

Cron support.

Fixing FeedRefreshTest.

Removed unused use statement (code style fix).

So far I found one small issue: for entity types for which no non-configurable action plugins exist the setting 'update_non_existant' does not show up. This is for example the case for taxonomy. This made sense in one of the earlier patches, but at a certain point I added a built-in delete operation (which is not an action plugin) because the existing actions plugins that perform delete operations don't do the deletions themselves: instead they collect which entities should be deleted and redirect the current user to a confirmation page. That results into nothing when an action is applied during cron runs: for the cron user it doesn't make sense to redirect it to a confirmation page.

I have running the patch for about two weeks now on a staging website, and encountered two things:

1. For entity types without non-configurable action plugins, the option didn't show up (as I already reported in #21). This is for example the case for taxonomy terms.
2. Under unknown circumstances the clean state doesn't get an entity type assigned. I couldn't figure out yet when, only noticed a lot of error messages in the log: 'The clean state does not have an entity type assigned.'.

I fixed the two issues above in the attached patch. Since I do not know the cause of the second issue, I did not write a test for it. I do know however that the error no longer got reported after making the change in CleanState::nextEntity().

Small cleanups (mostly in tests).

Great, all tests still pass! Committed #24.