Fix broken torso twist regression while updating weapon panel heat.

[HoneySkull]

This PR is to fix to update #5910 which inadvertently broke torso twisting when fixing #5902.

The reason #5910 broke torso twisting is because the call to re-display mech also resets the weapon list model.

This fix uses the same approach but remembers the currently selected unit's selected weapon and then resets it when clearing attacks.

Fixes #5650 again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.91%. Comparing base (7e11fae) to head (180826d).
Report is 13 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5944      +/-   ##
============================================
- Coverage     28.91%   28.91%   -0.01%     
+ Complexity    13945    13943       -2     
============================================
  Files          2539     2539              
  Lines        268383   268400      +17     
  Branches      47937    47943       +6     
============================================
- Hits          77607    77606       -1     
- Misses       186816   186828      +12     
- Partials       3960     3966       +6     

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[HammerGS]

I’ll catch up the merge commit later this evening when home.