Use correct CharSet for WNDCLASSEX

And prepend `Avalonia-` to window class. Looking in Spy++ I noticed that our window class name was garbage because the `CharSet` on `WNDCLASSEX` was wrong. Fix that and include "Avalonia" in our window classes.

src/Windows/Avalonia.Win32/Embedding/EmbeddedWindowImpl.cs

@@ -4,7 +4,6 @@
 using System;
 using System.ComponentModel;
 using System.Runtime.InteropServices;
-using System.Windows.Forms;
 using Avalonia.Platform;
 using Avalonia.Win32.Interop;
 

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1098,7 +1098,7 @@ namespace Avalonia.Win32.Interop
             }
         }
 
-        [StructLayout(LayoutKind.Sequential)]
+        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
         public struct WNDCLASSEX
         {
             public int cbSize;

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -641,7 +641,7 @@ namespace Avalonia.Win32
             // Ensure that the delegate doesn't get garbage collected by storing it as a field.
             _wndProcDelegate = new UnmanagedMethods.WndProc(WndProc);
 
-            _className = Guid.NewGuid().ToString();
+            _className = "Avalonia-" + Guid.NewGuid();
 
             UnmanagedMethods.WNDCLASSEX wndClassEx = new UnmanagedMethods.WNDCLASSEX
             {