Timeline should support action indicators, text from spec:
The left-most column of the Timeline palette is used to show which elements have
actions on them.
• A row with an A on it indicates that the element has one or more actions on it.
• A row with a down-triangle in this column indicates that one or more
descendant elements have actions on them.
• A row with a right-triangle in this column indicates that the component on this
row has one or more actions inside it.
Elsewhere in the interface, a green item indicates an action on the master slide,
while a light gray item indicates an action specific to this slide.