feat: LEAP-1151: Show region indices on regions

[hlomzik]

Display region index from Outliner on regions itself, so it will help to find regions visually.
Index is stored in region and updated every time the order changes (new region added, order direction is changed)

PR fulfills these requirements

• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

With lot of regions and/or with overlapping regions it's hard to select exact region you need. This fix adds region indices from regions tree, so you can select the region you need precisely in regions tree, without interaction with image, just by looking for the number.

Does this change affect performance?

It writes region_index on every regions tree change, but that's a volatile property + it's updated only if index is changed, so should be a very small effect.

Does this PR introduce a breaking change?

• Yes, and covered entirely by feature flag(s)
• Yes, and covered partially by feature flag(s)
• No
• Not sure (briefly explain the situation below)

What level of testing was included in the change?

• e2e
• integration
• unit

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