Fix exporting MUL files from MML

[pavelbraginskiy]

Previously units created in MML were not given IDs, which created problems when those units were added to a game for MUL file export.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.17%. Comparing base (ca998fe) to head (a517a8a).
Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1629      +/-   ##
===========================================
- Coverage      2.17%   2.17%   -0.01%     
  Complexity      209     209              
===========================================
  Files           266     266              
  Lines         30758   30760       +2     
  Branches       5259    5260       +1     
===========================================
  Hits            669     669              
- Misses        29932   29934       +2     
  Partials        157     157              

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