Add "Save entity" derivative

Does that mean we replace the existing entity save action?

Started, see progress in PR.
Work in progress...

needs review. See the PR in description

resolved

PR issues are answered.
Only few errors in Travis left. Like: Drupal\Component\Plugin\Exception\PluginNotFoundException: The "rules_entity_save" plugin does not exist.

Only one failure left:

There was 1 failure:
1) Drupal\Tests\rules\Integration\Engine\IntegrityCheckTest::testIntegrityCheck
Failed asserting that 1 matches expected 0.
/home/travis/build/fago/rules/tests/src/Integration/Engine/IntegrityCheckTest.php:48

Is it really desirable, from a UI/end user standpoint, to have a dozen different "Save content", "Save comment", "Save taxonomy term" etc. actions showing up in the select list, rather than just one generic "Save entity" ?

I think we're getting to the point where there are far too many conditions to choose from, and far too many actions to choose from. So simplifying things by providing one "Save" action rather than one for each entity may be preferable.

Added #3255559: Re-label entity create action derivatives as one concrete example of where having a derivative action for each entity type causes confusion. In that case, the number of entities defined on the system resulted in a list of actions where most of the entries were derivatives. Those obscured the limited number of built-in Rules actions, and all those derivatives misled the original poster.

Marking this as "won't fix" for the reasons explained above and in #3255559: Re-label entity create action derivatives