Allow VisualScript nodes other than built-in methods and members to have descriptions

**Godot version:**
3.1

**Issue description:**
Currently only built-in methods and members can have descriptions. It would be useful if VisualScript specific nodes like the flow control or the basic type manipulation could have descriptions too. There should be a way to change the description of a custom visualscript node, too. A `_get_description` function would be great.