Navigation: Implement proper getOriginalElement() for Kotlin declarations.

Run find usages tests with library for both original and navigation elements. Drop duplicating tests. Simplify test class.
TODO: Some usages are not found yet since light methods built from library files do not retain original declarations

compiler/frontend/src/org/jetbrains/kotlin/psi/JetDeclarationStub.java

@@ -52,16 +52,16 @@ abstract class JetDeclarationStub<T extends StubElement> extends JetModifierList
         return super.getParent();
     }
 
+    @Override
+    public PsiElement getOriginalElement() {
+        KotlinDeclarationNavigationPolicy navigationPolicy = ServiceManager.getService(KotlinDeclarationNavigationPolicy.class);
+        return navigationPolicy != null ? navigationPolicy.getOriginalElement(this) : this;
+    }
+
     @NotNull
     @Override
     public PsiElement getNavigationElement() {
         KotlinDeclarationNavigationPolicy navigationPolicy = ServiceManager.getService(KotlinDeclarationNavigationPolicy.class);
-        if (navigationPolicy != null) {
-            JetElement navigationElement = navigationPolicy.getNavigationElement(this);
-            if (navigationElement != null) {
-                return navigationElement;
-            }
-        }
-        return this;
+        return navigationPolicy != null ? navigationPolicy.getNavigationElement(this) : this;
     }
 }

compiler/frontend/src/org/jetbrains/kotlin/psi/KotlinDeclarationNavigationPolicy.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.psi
 
-public trait KotlinDeclarationNavigationPolicy {
-    public fun getNavigationElement(declaration: JetDeclaration): JetElement?
+public interface KotlinDeclarationNavigationPolicy {
+    public fun getOriginalElement(declaration: JetDeclaration): JetElement
+    public fun getNavigationElement(declaration: JetDeclaration): JetElement
 }