Future-proofing: change Alignment interface to abstract class and add a...

Future-proofing: change Alignment interface to abstract class and add a parameter for measurementType.

Also included bugfix from Winson: arcs wre not being cleared in invalidateStructure() so
state wasn't properly cleared when children were added after the first layout operation.

Change-Id: I7b46eadd955bd74b7bb3e4dad2f47ed3f092ff63