Bug/fix pinch and grab detection event subscription and grab detection logic

[MaxPalmer-UH]

Summary

Please do this before merging #1685

The PinchDetector and GrabDetector scripts were exposing properties for OnGrabStart/OnGrabbing/OnGrabEnd (same for pinch equivalents) as readonly. These could not be subscribed to.

Also the grab detection logic was inverted.

Contributor Tasks

• Create or edit test cases here
• Add a CHANGELOG entry for this change.
• Ensure documentation requirements are met e.g., public API is commented.
• Consider any licensing/other legal implications for this MR e.g. notices required by any new libraries.

Reviewer Tasks

• Code reviewed.
• Non-code assets e.g. Unity assets/scenes reviewed.
• All tests must be ran and cover all scenarios (If not, add new tests to the cycle and run them).
• Documentation has been reviewed.
• Approve with a comment of any additional tests run or any observations.

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[rblenkinsopp]

Happy with this but have included some suggestions for how to make the code a little more concise and readable, will approve if you want to leave it as is however.

[MaxPalmer-UH]

Happy with this but have included some suggestions for how to make the code a little more concise and readable, will approve if you want to leave it as is however.

I'll make the change. Nicer to have concise, more modern code.

[MaxPalmer-UH]

@rblenkinsopp I've pushed those changes now.

[rblenkinsopp]

Happy with this now