Changed the default icon ID to `32512` to be in line with the Windows default.

[zedseven]

Fixes mxre/winres#28.

I think it's absolutely reasonable to change the default to match what Windows expects as the default. Users of this crate will expect winres to be a canonical source of information, and using a default value of 1 just because other exes weren't using the Windows default doesn't make sense to me.

I also found a Windows property called DefaultIconResourceId, and it's also set to 32512: https://docs.microsoft.com/en-us/previous-versions/windows/desktop/cc982301(v=vs.85)