Automatically create a canvas when UI gameobjects are added

[Cameron-Micka]

Describe the problem

Adding UI GameObjects should automatically create a parent canvas when one is not selected. These menu items in particular:

Describe the solution you'd like

Just like how built-in UnityUI components act. When a canvas is not in the current selection one is made: https://github.com/Unity-Technologies/uGUI/blob/5ab4c0fee7cd5b3267672d877ec4051da525913c/UnityEditor.UI/UI/MenuOptions.cs#L143

Describe alternatives you've considered

Live with existing functionality.

Additional context

n/a

[Cameron-Micka]

See this PR for ways to reflect this functionality: https://github.com/microsoft/MixedRealityToolkit-Unity/pull/10950