Fixing issue where the Object Manipulation example fails in Unity 2018.3

[small-potatoes]

Fixing issue where the Object Manipulation example fails in Unity 2018.3 due to the order of calls to OnEnable and OnAwake (1) being different than previous versions of the editor and (2) exposing an initialization bug in ManipulationSystem.cs. This commit refactors ManipulationSystem.cs to allow access to static member ManipulationSystem.Instance before any calls to non-static member Awake.

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[fredsa]

Fixes #502