Move Win32 file dialog defs into type, const, and proc sections

[treeform]

Summary

• Move OPENFILENAMEW, the OFN_* flags, and GetOpenFileNameW / GetSaveFileNameW into the existing type, const, and imported-proc sections in windefs.nim.
• Drop the extra importc on those two procs so they inherit importc, stdcall from the single file-wide push/pop, same as the other Win32 APIs.

Test plan

• nim c examples/filedialogs.nim, examples/alertdialog.nim, and tests/test.nim
• Run examples/filedialogs.exe: O opens, S saves, cancel returns empty paths
• Skim the windefs.nim section order (types, then consts, then one importc proc block)

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