Keeps original tile-data around so that tile changes can be picked up

[philtweir]

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of Change

Keeps the original tile data in the data dict that will initialize the "edited" tile, so that (e.g.) tile-save receivers can see from the new tile's tile._original_data member, what was in there (and, consequently, what has changed).

Issues Solved

Closes #

Checklist

• I targeted one of these branches:
  • [x] dev/7.6.x (under development): features, bugfixes not covered below
  • [ ] dev/7.5.x (main support): regressions, crashing bugs, security issues, major bugs in new features
  • [ ] dev/6.2.x (extended support): major security issues, data loss issues
• [ ] I added a changelog in arches/releases
• [ ] I submitted a PR to arches-docs (if appropriate)
• [ ] Unit tests pass locally with my changes
• [ ] I added tests that prove my fix is effective or that my feature works
• [ ] My test fails on the target branch

Accessibility Checklist

Developer Guide

Topic	Changed	Retested
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Ticket Background

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• Found by: @
• Tested by: @
• Designed by: @

Further comments

[philtweir]

Can add a test here as well, but keen to get an OK in principle first!