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 jquerygh-1793

Author: borgboyone