Importing objects from .qpdata including TMA cores can result in an 'invisible' TMA grid

[petebankhead]

Bug report

Describe the bug
Importing objects from a .qpdata file containing TMA cores can result in the cores being brought over, but not properly assigned to a TMA grid. As a result, they are 'selectable' and appear under the 'Hierarchy' tab, but are otherwise not visible.

To Reproduce
Steps to reproduce the behavior:

1. Create a project with 2 images
2. Add a TMA grid to one (e.g. Specify TMA grid)
3. Save the data
4. Open the second image
5. Drag the .qpdata file from the first image onto the QuPath viewer
6. See the weird hierarchy behavior

Expected behavior

1. If there is a TMA grid on the second image already, offer to replace it with the one from the .qpdata file
2. If there is no TMA grid on the second image, import it 'properly'

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: All
• QuPath Version: v0.4.3 (and likely earlier)

Additional context
See https://forum.image.sc/t/tma-core-missing/85571

[petebankhead]

Now 'fixed' in the sense that TMA cores are filtered out when importing objects. That seems better than importing them very very badly.