Visualize center of mass

[chapulina]

Like on Gazebo-classic, it should be possible to visualize links' CoMs so users can debug their models and simulations.

On classic, CoM visuals consist of a sphere scaled proportionally to the link's mass, and lines stretching to the link's bounding box. The sphere represents a uniform ball of lead with the same mass, so if it is as big as your model, your model is as dense as lead!

CoM visualization could be toggled from:

• A world's right-click context menu: toggles all CoMs in the world
• A model's right-click context menu: toggles all CoMs in the model
• A link's right-click context menu

We could also consider adding toggle buttons to an entity's component inspector.

Here's classic's CoMVisual implementation as a reference.

And here's the PR where the visuals were added to Gazebo-classic.

Related to #111 and ros-visualization/rviz#1204.

[ahcorde]

Ign-rendering6 adds this feature gazebosim/gz-rendering#345

next step create a PR on ign-gazebo

FYI @atharva-18

[chapulina]

#903