Improve editing performance

[ishubin]

• implement cache for items outline points
• improve adding item performance
• deleting an item

Notes:

• When adding item to large doc:
  • it runs a full reindex in case the item was dragged into another item while it should only add points to pins and outline spatial indices (in itemCreationDraggedToSvgEditor)
  • indexItemPins -> forEach -> ... ->myMath.worldPointInArea function took 37 ms and it caused GC to spend 33ms. Looks like it generates too many objects
  • scrollToSelection function took 72 ms