Use unmodifiable collection in UnitSeparator in Swing Event Thread Task

[DanVanAtta]

Fixes #7066 UnitSeparator#categorize:103 - ConcurrentModificationException

This fix avoids concurrent modification to a list by creating a copy
of the list before we operate on the list in the swing even thread.
We go on to fix any other data issues that could change between when
we issue the task to the swing event queue by computing all data
the task needs before creating the task.

Functional Changes

[] New map or map update
[] New Feature
[] Feature update or enhancement
[] Feature Removal
[] Code Cleanup or refactor
[] Configuration Change
[x] Problem fix
[] Other:

Testing

• Did a quick smoke test to make sure the placement panel rendered during a single player game and did not render if no units were purchased

Screens Shots

Additional Notes to Reviewer

Release Note

FIX|Potential game crash during calculation of units that belong in the units-to-place panel

[codeclimate]

Code Climate has analyzed commit a0ee0b1 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 26.4% (0.0% change).

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