MHQ RFE 5604: save refit files from MHQ MekLab tab (MML side)

[Sleet01]

Supporting code changes to allow MHQ to utilize the existing unit saving code within its MekLab tab.
Changes include:

• Moving much of the MTF / BLK file saving code from the MML menu bar UI code out to a separate class;
• Creating a new interface based on the existing save functionality to facilitate using this new class with MHQ;
• Separating the entity-related code from the file-save-related code as much as possible.

Testing:

• Ran all 3 projects' unit tests
• Loaded and saved multiple units of various kinds with the updated code.

NOTE: this PR only sets up the code needed by the MHQ PR, which will close the above RFE issue.
The other PR is here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.14%. Comparing base (a2abcdd) to head (d74b4e9).
Report is 13 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1678      +/-   ##
===========================================
- Coverage      2.14%   2.14%   -0.01%     
  Complexity      209     209              
===========================================
  Files           270     271       +1     
  Lines         31247   31261      +14     
  Branches       5328    5330       +2     
===========================================
  Hits            670     670              
- Misses        30420   30434      +14     
  Partials        157     157              

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[Sleet01]

Just realized that the new files need headers; converting to draft for now.

[HammerGS]

Co-Pilot checked and not issues found.