HITL - Disallow placing objects onto held objects.

[0mdc]

Motivation and Context

This change disallows placing objects onto objects held by other agents.

How Has This Been Tested

Tested in multiplayer HITL.

Types of changes

• [Bug Fix]

Checklist

• [x] My code follows the code style of this project.
• [x] I have updated the documentation if required.
• [x] I have read the CONTRIBUTING document.
• [x] I have completed my CLA (see CONTRIBUTING)
• [x] I have added tests to cover my changes if required.

[0mdc]

@zephirefaith I'll soon be hooking up the UI to an event system to capture all user-generated events. I'll give you pointers.