Add std::less specialisation

For std::shared_ptr<const Plugin>, so that the order of elements
returned by GameCache.GetPlugins() is dependent on filenames only.

src/backend/game/game_cache.h

@@ -77,4 +77,14 @@ private:
 };
 }
 
+namespace std {
+template<>
+struct less<std::shared_ptr<const loot::Plugin>> {
+  size_t operator() (const std::shared_ptr<const loot::Plugin>& lhs, 
+                     const std::shared_ptr<const loot::Plugin>& rhs) const {
+    return lhs->GetLowercasedName() < rhs->GetLowercasedName();
+  }
+};
+}
+
 #endif