Helper/Item Overlap Detection Out of Alignment After Scroll

[A. Wells]

After a sortable container is scrolled via helper drag activity, movement of the helper results in the placeholder being rearranged to an item that is not under the helper but to an item the same distance away as the scrolled distance. Continued helper drag activity results in a reset of positions and overlap detection returns to normal until scrolling is commenced again.

​http://stackoverflow.com/questions/41261182/jquery-ui-sortable-drop-area-is-not-detected-properly-if-scrolling-and-connectw

[Scott González]

In c866e45:

Sortable: Fix various scrolling issues

• Created _scroll extension point and migrated scroll code from _mouseDrag
• Cleaned up logic for scrolled
• Fixed appendTo functionality to match documentation
• Remove unnecessary function calls
• Move set-up position functions to appropriate place
• Base scrollParent on placeholder and not helper
• Update scrollParent when switching containers

Fixes #3173
Fixes #15165
Fixes #15166
Fixes #15167
Fixes #15168
Fixes #15169
Fixes #15170
Closes gh-1793