Support action: animation for slide and widget player

[avanwinkle]

This PR adds support for action: animation on slide and widget players to trigger animations on an MPFSlide or MPFWidget's attached animation_player node.

The syntax looks like this:

slide_player:
  item_highlighted:
    attract:
      action: animation
      animation: item_highlight_anim

In this example, the item_highlighted event would trigger the attract.tscn slide's AnimationPlayer node to play its "item_highlight_anim" animation.

This change is inspired by @albano353's carousel animation proposal in #23, taking that idea and extracting it to be broadly applicable. All MPFSlide and MPFWidget nodes already support an attached AnimationPlayer(for enter/exit/active animations) so the change to support the action: animation functionality is pretty simple.

Tested on a local game project using the above code snippet.

Note: this change requires a corresponding config_spec.yaml change, which has been merged to the MPF 0.80.x branch. Please pull the latest MPF to test.