Support YRpp spawner's MP saves

[Metadorius]

Also see CnCNet/yrpp-spawner#48. You should test this PR with any recent YRpp spawner version with MP saves, should work.

Scenarios to test:

• quicksave
• save game trigger action

Conditions:

• online with YRpp spawner (should work)
• online with old cncnet5.dll / old YRpp spawner without support for MP saves

[github-actions]

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• compiled-dll-0b1656869a15427f975ff17a2b7cdcafd668b147.zip
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[Metadorius]

Took over the XNA client's handling of savegames. This should fix the "saving too fast" issue.

Please verify all previously faulting scenarios, and in addition to that also verify if the saves in skirmish/campaign are alright still, also verify that saves are alright with old cncnet5.dll.