Steps towards SBF, player turns and stuff

[SJuliez]

This PR is a further small step towards SBF game handling. While having to look closely at our classes, I am using the opportunity to do some refactoring (or better: reorganization), making our existing classes smaller and seeing how to reuse them for SBF games.
This PR mainly

• adds a PlayerTurn interface to generalize player turns (GameTurn is specific to TW games)
• moves out GameTurn's inner subclasses
• moves up some general phase handling to AbstractGameManager and moves out the specific implementations into handler classes, also for GameManager (cutting its length by over 1000 lines ... i.e. around 3% lol); this requires some access modifier changes to methods in GameManager

As in the last PRs for SBF I ask to leave in the loads of commented code in the SBF classes so I don't have to re-copy them later to see what's to be done. There are no intentional changes for standard games in here.

For review, there is a lot of code movement in here, so it's not quite as much as it looks like. Still, it's another big one. To test it I had two Princesses have a royal rumble and it seemed to work well.

[codecov]

Codecov Report

Attention: Patch coverage is 1.87919% with 731 lines in your changes missing coverage. Please review.

Project coverage is 29.29%. Comparing base (a88ea44) to head (ae6f1a2).
Report is 12 commits behind head on master.

❗ Current head ae6f1a2 differs from pull request most recent head 3706e70

Please upload reports for the commit 3706e70 to get more accurate results.

Files	Patch %	Lines
megamek/src/megamek/server/TWPhaseEndManager.java	1.67%	176 Missing ⚠️
.../src/megamek/server/TWPhasePreparationManager.java	1.73%	170 Missing ⚠️
megamek/src/megamek/server/SBFPhaseEndManager.java	0.00%	54 Missing ⚠️
...src/megamek/server/SBFPhasePreparationManager.java	0.00%	44 Missing ⚠️
...amek/src/megamek/server/GameManagerSaveHelper.java	7.14%	39 Missing ⚠️
megamek/src/megamek/common/UnloadStrandedTurn.java	0.00%	37 Missing ⚠️
megamek/src/megamek/common/EntityClassTurn.java	0.00%	31 Missing ⚠️
...egamek/src/megamek/server/AbstractGameManager.java	6.66%	28 Missing ⚠️
megamek/src/megamek/server/SBFGameManager.java	0.00%	27 Missing ⚠️
megamek/src/megamek/common/Game.java	0.00%	18 Missing ⚠️
... and 28 more

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5541   +/-   ##
=========================================
  Coverage     29.28%   29.29%           
- Complexity    13758    13771   +13     
=========================================
  Files          2443     2459   +16     
  Lines        262494   262554   +60     
  Branches      46974    46963   -11     
=========================================
+ Hits          76875    76917   +42     
- Misses       181766   181796   +30     
+ Partials       3853     3841   -12     

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[Sleet01]

Some naming questions, otherwise looks good (I didn't spend much time on SBF stuff as I believe it's still in flux).