New wxEmbed sample

[Adam Lock]

I've just checked in a work-in-progress wxWindows based sample in 
mozilla/embedding/tests/wxEmbed. It's early days yet, but wxEmbed is 
meant to exercise the Gecko embedding APIs in more diverse ways than the 
other embedding test apps. So it might eventually contain various test 
scenarios (e.g. browser, mail reader, chat, editor etc.). So far it's 
just a simple browser and a mail-like client and is still in development.

Hopefully some interesting reusable classes (and not just for wx) will 
also fall out of it. People trying to write protocol handlers or chrome 
objects might already like to look at the reusable Gecko* classes. These 
too are in development.

Build instructions are in README.txt, but it only builds on Win32 at the 
moment. People looking for a proper wxWindows solution should probably 
check out wxMozilla, from which this code is *not* derived.

Adam