unify(common-rts): Move low-risk Common RTS files to Core

[Jaredl-Dev]

This moves the remaining shared low-risk Common RTS files into Core:

• Handicap.h / Handicap.cpp
• MissionStats.h / MissionStats.cpp
• PlayerList.h / PlayerList.cpp
• ResourceGatheringManager.h / ResourceGatheringManager.cpp
• Science.h / Science.cpp
• ScoreKeeper.h / ScoreKeeper.cpp

The moved files are code-identical between both games except for the product-name license banner. ResourceGatheringManager.cpp also corrects a comment spelling.

Verification

• Verified all Core files byte-match their Zero Hour sources.
• Win32 Release build succeeds.
• VC6 Release build succeeds.

[Skyaero42]

ResourceGatheringManager.cpp needs to have a merge first (separate commit) before being merged.

[xezon]

How about merge all the Common/RTS files first, then move them all?

[Jaredl-Dev]

How about merge all the Common/RTS files first, then move them all?

I will revise this PR when I have the opportunity.