SyncList: keep old OnChange parameters to not break projects

[miwarnec]

we've had 10 years worth of projects hook into SyncLists for changes with (oldItem, newItem). reducing this to one parameter that is sometimes oldItem, sometimes newItem introduces too much pain when upgrading.

this will save people thousands of dollars in upgrade costs.

[MrGadget1024]

You'll have to obsolete the current and fix all the tests to work for both.

I strongly disagree with this PR.

[MrGadget1024]

Better to put Callback back in and not break everyone who has already adapted to OnChange and the new actions