Allow saving a MUL file from the queue-to-print dialog.

[pavelbraginskiy]

Closes #1592.

In addition to letting you reorder the units in a MUL file, this also allows you to make a MUL file directly in MML, without involving MM, although you'll still need to involve MM to set pilot data or wire C3.

This PR also enables you to add units from cache when printing from a MUL file to add additional units to it, but the additional units won't have pilot data.

Eventually this dialog should have a way to customize units directly, but that's a more long-term goal.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.65%. Comparing base (126e393) to head (91bb560).
Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1595      +/-   ##
===========================================
- Coverage      0.65%   0.65%   -0.01%     
  Complexity       64      64              
===========================================
  Files           266     266              
  Lines         30523   30545      +22     
  Branches       5203    5206       +3     
===========================================
  Hits            201     201              
- Misses        30282   30304      +22     
  Partials         40      40              

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[Sleet01]

LGTM. Certainly I prefer re-using existing code in a simple and clear fashion over refactoring all the GUI stuff!

[HammerGS]

Thank you for this!