Add C# mimetype for KDE 4. (This is a temporary solution
until fd.o's XDG MIME-type database is supported.)

The "x-csharp" MIME-type is what XDG also uses
(see <a href="https://bugs.freedesktop.org/show_bug.cgi?id=231">https://bugs.freedesktop.org/show_bug.cgi?id=231</a>;)
and does not yet have a corresponding icon in the
Crystal or Oxygen iconsets.

There won't be a C# mimetype for KDE 3.x anymore.