Fix sorter performance

[mohamedsalem401]

Changes

Most changes are focused on performance enhancement. The following operations were expensive:

• Repositioning the placeholder
• Recomputing the drop position
• Dragging from the blocks (adding the block to the shallow wrapper each time was costly)
• Checking if a given component is displayed or not
• Calling canMove every time the mouse moved
• On canvas scroll handler was not being cleared

Improvements

• Repositioning the placeholder: Now happens every 50 milliseconds.
• Recomputing drop position: Now happens every 20 milliseconds. Both calculations are 100% accurate for determining the final drop position.
• Shallow wrapper models: Cached for the entire dragging process.
• Component display check: Now cached as well.
• canMove: Called only when the user changes the index or target.
• Move the canvas scroll handler to the component sorter and clear it after sorting
• Improvements to bubbling from the mouse target to the first valid target.

These changes should improve the overall performance.