v0.16.0

• Add snapshotState<S>()
final state = spot<MyContainer>().snapshotState<MyContainerState>()
• Add snapshotRenderBox()
• Export WidgetPresence
• Add @useResult to .atMost(N), .atLeast(N), .amount(N) and .existsAtMostNTimes(N) to prevent missing assertions
• Fix existsAtLeastNTimes dumping the widget tree to console
• Fix image rendering with TimelineMode.always
• Add Timeline to /README.md
• Add act to /README.md