GraphEdit nodes max zoom, min zoom and zoom factor can't be changed

[timkrief]

Godot version:
Godot 3.2.1

OS/device including version:
Ubuntu 18.04.3 LTS

Issue description:
The GraphEdit node is limited to 3 zoom-ins or zoom-outs with a fixed zoom factor. After further inspection it seems like this was hardcoded around here. I think setting the min and max values as well as the scaling factor are supposed to be customizable.

Steps to reproduce:

• In any GraphEdit node try to zoom for multiple times, it will be limited.
  or
• In the editor, try to assign a large or small zoom value to a GraphEdit node, it will be limited

Minimal reproduction project:
Create a scene with a GraphEdit node.