IDEA-143920 Sort symbols in completion list in "natural" order

java/java-tests/testData/codeInsight/completion/normalSorting/NaturalSorting.java

@@ -0,0 +1,12 @@
+public class MainClass1 {
+
+  {
+    fu<caret>
+  }
+  
+  void fun1() {}
+  void fun10() {}
+  void fun2() {}
+
+}
+

java/java-tests/testSrc/com/intellij/codeInsight/completion/NormalCompletionOrderingTest.groovy

@@ -704,4 +704,8 @@ interface TxANotAnno {}
     checkPreferredItems 0, 'MainClass1', 'MainClass2', 'Maa'
   }
 
+  public void testNaturalSorting() {
+    checkPreferredItems 0, 'fun1', 'fun2', 'fun10'
+  }
+
 }

platform/lang-impl/src/com/intellij/codeInsight/completion/CompletionLookupArranger.java

@@ -60,7 +60,7 @@ public class CompletionLookupArranger extends LookupArranger {
     public int compare(LookupElement o1, LookupElement o2) {
       String invariant = PRESENTATION_INVARIANT.get(o1);
       assert invariant != null;
-      return invariant.compareToIgnoreCase(PRESENTATION_INVARIANT.get(o2));
+      return StringUtil.naturalCompare(invariant, PRESENTATION_INVARIANT.get(o2));
     }
   };
   static final int MAX_PREFERRED_COUNT = 5;