New graph for visualizing tracked detections

• Add overlay graphs to display both base detection and tracked detections.
• Replace absl::make_unique with std::make_unique (introduced in C++14).

Initial open source release

• First release to GitHub.
• New graphs: face blurring, face overlay, face pixelization, face sticker
  redaction.
• Magritte library API that simplifies common tasks by providing abstractions
  around the use of MediaPipe in many places.
• Added Android and desktop example apps.
• Added documentation and C++ codelab.