Improve performance of JS implementation of AlternativeLocationImpl

#getStringLocation reads the original file, but is called from the EDT,
so invocations must be kept to a minimum. What is more the method is
used by #compareTo and thus is potentially called often.

Two optimisations are applied:

- the result of #getStringLocation is cached and only calculated once
- #compareTo was rewritten to use the relative path and offset directly.
  Before relative path and the line number was used to create an
  ordering, as offset and line number are related, switching for
  ordering shoud have no outside result

webcommon/javascript2.editor/src/org/netbeans/modules/javascript2/editor/navigation/DeclarationFinderImpl.java

@@ -398,6 +398,7 @@ public static class AlternativeLocationImpl implements AlternativeLocation {
         private final int offset;
         private final DeclarationLocation location;
         private final IndexedElement element;
+        private String stringLocation;
 
         public AlternativeLocationImpl(IndexResult iResult) {
             this.iResult = iResult;
@@ -412,7 +413,10 @@ public ElementHandle getElement() {
             return element;
         }
 
-        private  String getStringLocation() {
+        private String getStringLocation() {
+            if(stringLocation != null) {
+                return stringLocation;
+            }
             int lineNumber = 0;
             int count = 0;
             List<String> asLines;
@@ -435,6 +439,7 @@ private  String getStringLocation() {
             if (lineNumber > 0) {
                 result = result + " : " + lineNumber; //NOI18N
             }
+            stringLocation = result;
             return result;
         }
 
@@ -452,7 +457,13 @@ public DeclarationLocation getLocation() {
         @Override
         public int compareTo(AlternativeLocation o) {
             AlternativeLocationImpl ali = (AlternativeLocationImpl)o;
-            return getStringLocation().compareTo(ali.getStringLocation());
+            String relPath1 = iResult.getRelativePath();
+            String relPath2 = ali.iResult.getRelativePath();
+            int comparison = relPath1.compareTo(relPath2);
+            if(comparison != 0) {
+                return comparison;
+            }
+            return offset - ali.offset;
         }
 
     }