Fix a couple property handler bugs

[jhaigh0]

Description of work

Initially this work was just to fix #37129, but I also found a bug where all the remaining "Fix" properties would be removed when removing a function from the same composite function.

Summary of work

• To fix the original issue, the parameter index needed to be retrieved from the top level composite function so the long prefix (f0.f0.f0) could be parsed properly.
• To fix the issue with removing the Fix sub properties, a check has been added to renameChildren to see if the parameter is fixed.
• I noticed that when adding the first function to a convolution, it sets all parameters to be fixed. When this gets initialised in the property handler, I've changed it to use the fix function so it is all more consistent
• I've also reduced some code reuse by changing fix to call addTie which now checks if the tie is static (aka a fix).

Fixes #37129

renameChildren definitely still contains a problem or two. I don't think it copes well if you had a bunch of nested composite function and wanted to tie across different levels, but this isn't a regression, so I will try and fix these in a later pr.

To test:

• Follow the instructions from the issue
• Clear the model
• Add two Gaussians
• Right click to fix the height of one of the Gaussians
• Remove the other Gaussian, the fix property should remain in place.

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[robertapplin]

Code looks good and the original crash no longer happens. I just had two questions, then happy to approve

[robertapplin]

Thanks for confirming - in that case happy to approve!