[bug-gettext] Support extracting named attribute parameters from C# using xgettext

[Robert Jørgensgaard Engdahl]

This is as much a feature-request as it is a bug report.

In C# one could write code like

```

[DisplayFormat(FormatString="Duration in minutes|{0:n0} min")]

public decimal DurationMinutes {get;set;}

```

and I would like the string "{0:n0} min" to be extracted as a msgid and "Duration in minutes" as msgctxt (GNOME glib context syntax is a practical approach when enhancing existing frameworks).

In C# preprocessor directives aren't as expressive as they are in C, so unfortunately

```

[DisplayFormat(FormatString=Noop("Duration in minutes|{0:n0} min"))]

public decimal DurationMinutes {get;set;}

```

will not compile.

I am using gettext in WPF (a bit late for that platform) and I am dedicated to make that a painless experience https://github.com/robert-j-engdahl/ngettext-wpf

I have some experience with C, and GNU tools, but have never contributed to any open-source C projects.