IDEA-104500 Gradle: Allow to reuse common logic for other external systems

1. External system-local settings are corrected;
2. Gradle toolwindow id change: JetGradle -> Gradle;
IDEA-104500 Gradle: Allow to reuse common logic for other external systems

Exclude ExternalSystemExtensions.xml from platform config because we don't want to enforce external-system-impl dependency at the ides which don't need it