#23 Add API for moving all sections to the right, starting with a specified one

Optionally, it can skip an item that is already within the range of
sections. The use-case for that is dropping an item onto a section, and
the local collisions not being resolved.
#23 _findLeftMostPositionForItem now stops at page boundary whenever column sections are enabled
#23 Refactor code in pullToLeft that places items starting with their leftmost limit.
#23 Collision resolving works differently in column sections mode

It tries to resolve the collisions locally and then pushes all the
content to the right by preserving section integrity.
#23 In column sections mode, collision detection only tries candidates within the same section