Tag::compare arguments should be const references

Besides being more correct, this fixes compilation errors with GCC 4.8
in KMail.

src/tag/tag.cpp

@@ -111,7 +111,7 @@ Akonadi::Tag Tag::saveToAkonadi(Tag::SaveFlags saveFlags) const
     return tag;
 }
 
-bool Tag::compare(Tag::Ptr &tag1, Tag::Ptr &tag2)
+bool Tag::compare(const Tag::Ptr &tag1, const Tag::Ptr &tag2)
 {
     if (tag1->priority < tag2->priority) {
         return true;
@@ -122,7 +122,7 @@ bool Tag::compare(Tag::Ptr &tag1, Tag::Ptr &tag2)
     }
 }
 
-bool Tag::compareName(Tag::Ptr &tag1, Tag::Ptr &tag2)
+bool Tag::compareName(const Tag::Ptr &tag1, const Tag::Ptr &tag2)
 {
     return (tag1->tagName < tag2->tagName);
 }

src/tag/tag.h

@@ -59,9 +59,9 @@ public:
     Akonadi::Tag saveToAkonadi(SaveFlags saveFlags = SaveFlags(TextColor | BackgroundColor | Font)) const;
 
     // Compare, based on priority
-    static bool compare(Ptr &tag1, Ptr &tag2);
+    static bool compare(const Ptr &tag1, const Ptr &tag2);
     // Compare, based on name
-    static bool compareName(Ptr &tag1, Ptr &tag2);
+    static bool compareName(const Ptr &tag1, const Ptr &tag2);
 
     qint64 id() const;
     QString name() const;