Tooltips IDs can't go above 0xFFFF.
Remove unused variables after eb191c09cc96450a4e62c3592a5697b407f6bc36.
Remove some overzealous constifications to avoid C4510 and C4610 warnings.

The language resources array is const already so there is no risk of unwanted modification.
Fix some typos.
Add missing file in MPCTestAPI.vcxproj.filters.
Update Changelog.txt.
Be consistent with the var type.