Revert "Revert: flatten ConfigurationPerRunnerSettings: remove intermediate JDOMExternalizable wrapper (c563d8f)"

This reverts commit 9a414cb1684dc7634d9cd1ebb869e6192413a8b6.
sort out nullability after run configuration settings refactoring
Feature-branches: checkout existing branches
Feature-branches: OpenTaskDialog refactored
Feature-branches: OpenTaskDialog refactored
IDEA-69667, IDEA-63367 Mark file or folder as ActionScript/Flex library in Project View context menu