Enable Image Tracker creation at runtime

[SimonDarksideJ]

While pre-processing image trackers is useful, it would also be beneficial to create image tracker references at runtime dynamically.

This would allow the following use cases:

• Downloading a tracked image and registering with the image tracker
• Capturing of new tracked images using the camera for identifying the placement of new content

[philip-lamb]

Fixed in https://github.com/artoolkitx/arunityx/pull/49