Allow opening unit and MUL files with MML by file association or drag-drop

[pavelbraginskiy]

Allows you to drag unit files (MTF/BLK) or MUL files from your OS's file explorer onto MegaMekLab:

You can drag them onto the startup GUI or almost anywhere onto the unit editing GUI.

I've tested, and crit-slot drag-and-drop still works without issues, the only thing to note there is that you cannot drag files onto a unit's crit table, which I'm sure no one will mind.

You can also ask your operating system to open these files with MML if you tell it the path to MegaMekLab.exe (or equivalent executable for your system):

Unit files will be opened to edit, while MUL files will be opened to print.

A new setting is added to determine if dropped MUL files should be printed or exported to PDF:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.16%. Comparing base (34486a0) to head (ac1c381).
Report is 10 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1648      +/-   ##
===========================================
- Coverage      2.17%   2.16%   -0.01%     
  Complexity      209     209              
===========================================
  Files           266     268       +2     
  Lines         30801   30911     +110     
  Branches       5266    5286      +20     
===========================================
  Hits            669     669              
- Misses        29975   30085     +110     
  Partials        157     157              

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[HammerGS]

Tested and seems to work. Very cool addition.