Support removing the actor, light, or model from the root

[nkoenig]

🎉 New feature

Summary

The Root object didn't have a mechanism to clear/remove the actor, light, or model. This PR adds a Clear function that similar to the other Clear functions.

Test it

Tests have been added.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[scpeters]

looks good; I suggested some small, optional improvements to the test

[scpeters]

Suggested change

	EXPECT_NE(nullptr, root.Actor());
	EXPECT_NE(nullptr, root.Actor());
	EXPECT_EQ(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Model());

let's check all three pointers, expecting at most one to be non-null

[scpeters]

Suggested change

	EXPECT_EQ(nullptr, root.Actor());
	EXPECT_EQ(nullptr, root.Actor());
	EXPECT_EQ(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Model());

[scpeters]

Suggested change

	EXPECT_EQ(nullptr, root.Model());
	EXPECT_EQ(nullptr, root.Actor());
	EXPECT_EQ(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Model());

[scpeters]

Suggested change

	EXPECT_EQ(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Actor());
	EXPECT_EQ(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Model());

[scpeters]

Suggested change

	EXPECT_NE(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Actor());
	EXPECT_NE(nullptr, root.Light());
	EXPECT_EQ(nullptr, root.Model());

[scpeters]

Suggested change

	EXPECT_NE(nullptr, root.Model());
	EXPECT_EQ(nullptr, root.Actor());
	EXPECT_EQ(nullptr, root.Light());
	EXPECT_NE(nullptr, root.Model());