Add signal for state transition in AnimationNodeStateMachine

[mysticfall]

Godot version:
master / 9af72f0

OS/device including version:
Manjaro Linux 17.1

Issue description: (This issue is a feature request)
Sometimes, it is needed to monitor state transitions in a state machine for animations (AnimationNodeStateMachine).

As Godot supports auto advancing between states, writing a custom event which fires upon invoking AnimationStateMachinePlayback.Travel() isn't really an ideal workaround.

To satisfy such a requirement, it'd be better to add a signal to notify state transitions in AnimationNodeStateMachine.

[clayjohn]

Feature and improvement proposals for the Godot Engine are now being discussed and reviewed in a dedicated Godot Improvement Proposals (GIP) (godotengine/godot-proposals) issue tracker. The GIP tracker has a detailed issue template designed so that proposals include all the relevant information to start a productive discussion and help the community assess the validity of the proposal for the engine.

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