StateTimeline: Support second time field for state end

[leeoniya]

StateTimeline panel supports state terminations when it encounters the start of the next state or a null value.

it can be difficult or impossible to create these null values after the needed state duration, but the data often contains an endTime field that we can use to do this null-insertion internally.

we should assume a second time field in the data is the endTime and do this automatically.