Adding an automatic fill feature to the center button in mask tab

[4rnaudB]

Checklist

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
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• I've added relevant tests for my contribution
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Description

Automatically fills the text boxes for center button in the mask tab with the current voxel positions.

Linked Issues

[jcohenadad]

@4rnaudB please always add a justification when you close a PR.

In light of #49, I suggest to reconsider the relevance of adding this feature, given that filling feature already exists in FSLeyes.

Also, before taking any action, we need to discuss as a team (@po09i should have a word to say)

[4rnaudB]

Ok, the automatic fill feature is done, but it was mostly to understand the code better. I closed the branch because I got problems trying to clean up my commits and don't want to push all this.