Plone.app.mosaic

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- Fix saving/editing custom layouts and remove TTW layouts editing in the controlpanel.
[petschki]

- Refactor resource bundles: use only one mosaic bundle.
[petschki]

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- Add upgrade steps to cleanup resource registry.
[petschki]

- Upgrade JS dependencies and GHA config.
[petschki]

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- Fix saving TinyMCE content when editing source code.
[petschki]

- Fix saving data in properties modal which got lost after saving mosaic page.
[petschki]

- generalization in modal positioning (top center)
[petschki]

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- Code cleanup in properties modal.
[petschki]

- Fix moving Tiles when there are `max-columns` tiles within a row.
[petschki]

- Layout column handle drag/drop fixes and cleanup.
[petschki]

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- Fixed properties modal.
[petschki]

- Update to mockup 5.0.0-alpha.18
[petschki]

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- Update to mockup 5.0.0-alpha.17 -> This fixes file upload in modal forms.
[petschki]

- Fixed old upgrade step that tried to set plone.lessvariables.
Ignore this on Plone 6, because it fails.
[maurits]