TMT constants not exposed

[jnm2]

I'm particularly interested in TMT_CONTENTMARGINS and TMT_SIZINGMARGINS for use with GetThemeMargins, but it might make sense to add the whole lot, split by Retrieval Function.

Ideally the iPropId parameter type would then change from int to ThemeMarginProperty or whatever the enum name would be.

https://github.com/AArnott/pinvoke/blob/6cefeae10ca58e60dba0a43b910e3afecd524be5/src/UxTheme/UxTheme.cs#L45-L52