Improve simple merge

[zhiltsov-max]

Summary

This PR implements missing item attribute merging functionality in the simple merging algorithm. Also, it improves image merging in some cases, including data-supplied and placeholder (metainfo-only) images.

• Allowed Image creation from just a size value. Now images can be built from array, or a function returning it, a path, or size info, including any mixed variants. The old restriction for size seems unnecessary.
• Added Image class docs
• Added image search in VOC XML-based formats
• Added image path equality checks in simple merge, when applicable
• Added item attribute merging in simple merge

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT