Return reference rather than copy

No need to return a const copy by default in GetItems().  The const
reference will be faster.  Thanks to @GyrosGeier for the suggestion

pcbnew/connectivity/connectivity_algo.h

@@ -146,7 +146,7 @@ public:
             m_items.push_back( aItem );
         }
 
-        const std::list<CN_ITEM*> GetItems() const
+        const std::list<CN_ITEM*>& GetItems() const
         {
             return m_items;
         }

pcbnew/connectivity/connectivity_data.cpp

@@ -695,9 +695,9 @@ static int getMinDist( BOARD_CONNECTED_ITEM* aItem, const VECTOR2I& aPoint )
 }
 
 
-bool CONNECTIVITY_DATA::TestTrackEndpointDangling( PCB_TRACK* aTrack, VECTOR2I* aPos )
+bool CONNECTIVITY_DATA::TestTrackEndpointDangling( PCB_TRACK* aTrack, VECTOR2I* aPos ) const
 {
-    std::list<CN_ITEM*> items = GetConnectivityAlgo()->ItemEntry( aTrack ).GetItems();
+    const std::list<CN_ITEM*>& items = GetConnectivityAlgo()->ItemEntry( aTrack ).GetItems();
 
     // Not in the connectivity system.  This is a bug!
     if( items.empty() )

pcbnew/connectivity/connectivity_data.h

@@ -229,7 +229,7 @@ public:
 
     void RunOnUnconnectedEdges( std::function<bool( CN_EDGE& )> aFunc );
 
-    bool TestTrackEndpointDangling( PCB_TRACK* aTrack, VECTOR2I* aPos = nullptr );
+    bool TestTrackEndpointDangling( PCB_TRACK* aTrack, VECTOR2I* aPos = nullptr ) const;
 
     /**
      * Function ClearLocalRatsnest()