[core] Expose getChildStart()/getChildEnd() to subclasses
[core] Expose setPendingScrollPositionWithOffset() to subclasses
[layouts] Rename forceCreateLanes() to resetLayoutState()
[core] Use non-deprecated onDetachedFromWindow()
[core] Add getAdapter() protected method to TwoWayLayoutManager
[core, layouts] Re-do update handing and implement lane invalidation

Fixes #95
Fixes #96
Fixes #123
Fixes #131
[core, layouts] Re-implement first-visible child handling
[layouts] Remove the now unnecessary resetLayoutState() method