[v3] Diagnostics support

[david-c-kline]

This issue has been migrated a new MRTK repository, and the status of this issue will now be tracked at the following location:

• https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/issues/308

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Implement a new diagnostics "system" approach.

• [x] separate visualization from data layer
• [x] hardware status (battery, etc) system
  • [x] battery level (critical, low, restored) events
• [x] basic visualizers
  • [x] frame rate, etc
  • [x] memory statistics
  • [x] battery level, etc
• [ ] Configuration UI

[stale[bot]]

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