Any tool in the toolbox, these define Synfig's modes.
States can be transitory ( state_stroke is responsible only for displaying/inputting the stroke as you draw it with another tool) or they can register themselves with the Toolbox and be 'sticky'.
I don't know yet, but there's a lot of them! Snap
Probably everything that can be undone.
?where is an action created / stored / executed?
Synfig has an event system, using GTK's event system, I think. Snap