[Mono-winforms-list] VisualStyles implementation based on GTK+

[Jonathan Pobst]

> On Windows, in the HKEY_CURRENT_USER\Software\Mono registry key, you need to add a string value named "VisualStyles" with the data "GTK+".
> On non-Windows platforms you can either use DLL mapping for the GTK+ libraries or disable the "#define Windows" line in GtkPlus.cs.

This is fine for your testing, but before we support this, we need to 
default this to non-Windows platforms, and we probably won't support 
running this on Windows.  (I can't think of a use-case for using GTK 
native styles on Windows, other than testing.)

Please commit.

Jonathan