ECA User: new action to switch back user account

Problem/Motivation

With the 2 action plugin SwitchAccount and SwitchServiceAccount an ECA model can switch user account and that will automatically be reverted after the action successors have been completed.

We also want to be able to explicitly switch back to the previous user account, e.g. if we switched to a higher privileged user to load an entity and afterwards want to check permission, we therefore need to switch back.

Proposed resolution

Provide an action plugin that switches back to the previous account. Be careful that we don't switch back more often than previously switched to a different account and make sure, that the cleanup interface for action plugins doesn't switch back again, if an explicit switch back happened before then.