IDEA-109629 Gradle: just imported project is shown multiple times in Gradle tool window until reopening of the project

Ensure that external config paths use the same slashes ('\' vs '/')
goto popups: sort items by matching degree (IDEA-104857, IDEA-89861)
fail on stub loading when indexed content length doesn't match the current one
IDEA-109645 External system: after importing of an external project, 'External system' tool window appears in every project until IDEA restart

1. Storing 'newly imported project' flag at project user data instead of global jvm properties;
2. Green code policy;