fix: more MSVC warnings

[Nerixyz]

Further working towards #4825.

Despite some rather simple fixes, the following was done:

• Defined _WIN32_WINNT to Windows 10 - this is what the inclusion of Windows.h defines. I'm not 100% sure if this is okay (e.g. looking at support for Window 7 and Windows 8 #4852)
• Re-enabled C4505 ('function': unreferenced local function has been removed), as we only need to declare static declarations (i.e. declarations for functions local to a translation unit) in headers as either inline or in a source file.
• Disabled C4458 (declaration of 'identifier' hides class member) as we have a rule of exclusively using this-> to access members.

[pajlada]

Small note on the passthrough thing, which is a good catch, otherwise looks good 👍