Add node started/finished signals for animation state machines

[a-johnston]

Closes godotengine/godot-proposals#10253.

The other half of #102165

This adds node_started(StringName) and node_finished(StringName) to AnimationNodeStateMachinePlayback. Grouped StateMachines have their signals propagated up to the parent state machine, with their name prefixed. For example, with the following state machine:

Outer (grouped):

Inner (grouped):

the signals on the root playback are:

State machine node started:  Start
State machine node finished: Start
State machine node started:  Outer/Inner/Attack1
State machine node finished: Outer/Inner/Attack1
State machine node started:  Outer/Attack2
State machine node finished: Outer/Attack2
State machine node started:  Hit1
State machine node finished: Hit1
State machine node started:  End

Combined with #102165 the signals fire in the following order:

State machine node started:  Start
Node notify: MyStateMachine Started
State machine node finished: Start
State machine node started:  Outer/Inner/Attack1
Node notify: OneShot Started
State machine node finished: Outer/Inner/Attack1
State machine node started:  Outer/Attack2
State machine node finished: Outer/Attack2
State machine node started:  Hit1
State machine node finished: Hit1
State machine node started:  End
Node notify: MyStateMachine Finished
Node notify: OneShot Finished