Blender Git Loki

UI: Don't show Windows file association operator in search on other OSes

The Windows-specific "Register File Association" operator would show in
the search menu of other platforms. Decided to not disable it at
compile-time, like we do it with "Toggle System Console" (another
Windows-only operator), because that would require workarounds for the
translation tools. Instead the operator poll function always returns
false on unsupported platforms now.