Restructured CLR reference creation.

winforms/src/toga_winforms/__init__.py

@@ -2,6 +2,18 @@
 
 import toga
 
+# Add a reference to the Winforms assembly
 clr.AddReference("System.Windows.Forms")
 
+# Add a reference to the WindowsBase assembly. This is needed to access
+# System.Windows.Threading.Dispatcher.
+#
+# This assembly isn't exposed as a simple dot-path name; we have to extract it from the
+# Global Assembly Cache (GAC). The version number and public key doesn't appear to
+# change with Windows version or the underlying .NET, and has been available since
+# Windows 7.
+clr.AddReference(
+    "WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
+)
+
 __version__ = toga._package_version(__file__, __name__)

winforms/src/toga_winforms/app.py

@@ -4,11 +4,11 @@
 import threading
 from ctypes import windll
 
-import clr
 import System.Windows.Forms as WinForms
 from System import Environment, Threading
 from System.Media import SystemSounds
 from System.Net import SecurityProtocolType, ServicePointManager
+from System.Windows.Threading import Dispatcher
 
 import toga
 from toga import Key
@@ -17,13 +17,6 @@
 from .libs.proactor import WinformsProactorEventLoop
 from .window import Window
 
-# A Reference to WindowsBase Assembly is needed for Dispatcher
-clr.AddReference(
-    # This Assembly String is very specific and helps clr to resolve Assemblies present in the GAC.
-    "WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
-)
-from System.Windows.Threading import Dispatcher  # noqa
-
 
 class MainWindow(Window):
     def winforms_FormClosing(self, sender, event):