unify(shell): Move Shell to Core

[Jaredl-Dev]

Requires #3075 for successful VC6 and Win32 profile builds.

This moves the shared Shell files into Core:

• Shell.h
• Shell.cpp
• ShellMenuScheme.h
• ShellMenuScheme.cpp

Generals and Zero Hour differ only by product-name comments and the RTS_PROFILE_LEGACY initialization profiling hook in Shell.cpp.

Generals gets:

• Stops the legacy init profiling range before opening the main menu, matching Zero Hour.

Verification

• Verified all four Core files exactly match the Zero Hour source.
• Win32 Release build succeeds.
• VC6 Release build succeeds.

[Jaredl-Dev]

Generals now inherits Profile::StopRange("init"), but unlike Zero Hour, it never calls StartRange("init"). Should it be added to Generals’ WinMain.cpp?