Show default value for properties on documentation

[vnen]

Godot version:
Godot 3.0 (a98e949)

Issue description:

The default value of class members are usually added to the description, but not all descriptions have it and the default is not always obvious. It may also be hard to find and easy to deviate from a standard.

It would be interesting if the default value came together with the name:

bool visible = true
     set_visible(value) setter
     is_visible() getter