Don't export objects disabled from rendering

[rcorre]

OS:

Linux 5.4.7-arch1-1

Godot version:

3.1.2.stable.custom_build

Blender version:

Blender 2.81 (sub 16)

Issue description:

I believe older versions of the exporter would automatically skip exporting objects not set to render.
I noticed as of my current version of blender there is a "Disable in Renders" flag (you have to click the filter above the scene tree to show it).
I'm not sure if this was re-added or if I just didn't know where the icon was in 2.80, but now that it is there (again?), we should skip exporting objects not set to render.

I know there's a flag to skip exporting hidden objects, but I'd like to have reference objects in my scene that won't get exported, without me having to remember to hide them every time I export.

Minimal reproduction project: