Cropping map by selection leaves Tiled to think that tilesets are still in use

[Ablu]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Crop map by a selection
2. Try to delete an now unused tileset
3. Warning occurs about tileset still being in use
4. Close file
5. Reopen file
6. Tileset can now be removed without warning

Expected behavior
Removing works without notification after the tileset is no longer used, without needing to reload

Specifications:

• Tiled Version: 1.10.2

[bjorn]

Right, this is likely due to "used tilesets" being cached for performance reasons. Seems like the cache is not getting invalidated in this case.