Single-instance with command line - a feature suggestion

[Ole Nielsby]

I posted the code at the CodeDump forum, with some suggestions
about what to do with it.

http://wxforum.shadonet.com/viewtopic.php?t=17881

It works for me as it is, but I don't quite know where to place it in
the wxWidgets trunk.

My favourite idea is that the stuff should be integrated in the wxApp
class as such, selectable/deselectable by a flag, and the default
implementation of the cmd line parsing callbacks should mimick
the Mac doc handling - this would strengthen the crossplatform-ness.