* Create unique identifiers where multiple gateways are in use
Resolving issue https://github.com/home-assistant/core/issues/134497
* Added migration function to __init__.py
Added migration function to execute upon initialisation, to:
a) remove the erroneously-added config)_entry added to the device (gateway B gets added as a config_entry to a device associated to gateway A), and
b) swap out the non-unique identifiers for genuinely unique identifiers.
* Added tests to simulate migration from bad data scenario (i.e. explicitly executing migrate_entity_unique_ids() from __init__.py)
* Ammendments suggested in first review
* Changes after second review
* Rewrite of test_migrate_config_entry_and_identifiers after feedback
* Converted migrate function into major version, updated tests
* Finalised variable naming convention per feedback, added test to validate config entry migrated to v2
* Hopefully final changes for cosmetic / comment stucture
* Further code-coverage in test_migrate_config_entry_and_identifiers()
* Minor test corrections
* Added test for non-tradfri identifiers
ee486c269c