setters and getters for visual script

**Godot version:**
3.1.1

**OS/device including version:**
Ubuntu 18.04

**Issue description:**
It would be nice to have ability to use setters and getters in visual script.
My use case: I'm working in a team with an artist that's slowly increasing it's capabilities in using Godot as an editor. The main tool this artist is currently using to create some small 'custom logics' is actually an AnimationPlayer. In some scenes I'm giving simple way to run some logic from AnimationPlayer by changing variable value in inspector (for example, `export var up_position:bool` value of elevator).
Recently I wanted to introduce this artist to VisualScript, for the start to give her ability to create functions that are calling different animations from models she is providing... It was quite natural for her that she would like to expose such 'animation' to the inspector so she could use it from AnimationPlayer that's in parent scene