Symbol Editor: clear IS_NEW flag after placing a pin

IS_NEW flag needs to be cleared after a pin is placed, otherwise further
move operations will keep adding the pin to the edited symbol.

eeschema/tools/lib_pin_tool.cpp

@@ -181,6 +181,7 @@ bool LIB_PIN_TOOL::PlacePin( LIB_PIN* aPin )
             CreateImagePins( aPin );
 
         part->AddDrawItem( aPin );
+        aPin->ClearFlags( IS_NEW );
     }
 
     // Put linked pins in new position, and clear flags