Fix: Remove unnecessary properties for tile/grid layout (fixes #314)

[swashbuck]

Fix #314

Fix

• Ensures that the _graphic property on the main component is not required when using the tile / grid layout.
• Update example.json to remove unnecessary properties for the tile / grid layout: _top, _left, and _imageAlignment for each item.
• Update example.json to add clarity between the different layout types and to add more realistic image file names.
• Update README.md to add notes about tile/grid properties.

[cahirodoherty-learningpool]

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