c# -> vb custom event improvements

[Alexilia]

fix conversion of custom event with backing field

Problem

#435

Solution

[GrahamTheCoder]

Thanks. I'm not quite sure whether it'll do any harm marking all private delegates as events. At some point we might need to do something cleverer, but this seems like a great first step.

If you're planning to contribute quite a bit of C# to VB conversion code, it's probably worth being aware of #8 (comment)
Paul converted the converter to VB, then worked on some improvements. At some point I was intending to convert his code back to C# and diff them (which will be quite messy) to see what could be merged back. I haven't got around to it yet, but it's possibly a good place to look for hints at minimum.