Helper/Item Intersection checking should not occur if the helper is not inside of a container

[A. Wells]

It appears that the code that kicks off helper and item intersection checking engages even when the helper is outside of a container. Performance would be improved if the container intersection check is performed first in the _mouseDrag function and the currentContainer variable (potentially innermostContainer as well) is immediately set to null if no intersection is found. Scrolling and rearrangement of the placeholder via item loop can then be bypassed accordingly.

[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