No way to get native window handle pointer

[rthomasv3]

Currently the webview_get_window binding is not implemented.

On Windows this is no big deal - you can just use Process.GetCurrentProcess().MainWindowHandle, but this won't work on other platforms (always returns 0).

Getting the native window handle is required for implementing platform-specific features in downstream applications, such as enabling spell checking on Linux via WebKit2GTK APIs.

This change shouldn't require new build of the native libs, just a small change to add the binding.

Proposed changes:

1. Add webview_get_window() in Bindings.cs
2. Add public GetWindow() method to Webview.cs that returns the native window handle

[geaz]

Closed by #53