Asynchronous IPC: Initial implementation

This commit adds a mixin which sets up a modal operator which:
* Starts a subprocess
* Polls for messages from that subprocesses

pkg_ui.py

@@ -16,8 +16,8 @@ class USERPREF_PT_packages(bpy.types.Panel):
         layout = self.layout
 
         row = layout.row()
-        row.prop(context.window_manager.PackageManagerSettings, "url")
-        row.operator(pkg_ops.PACKAGE_OT_fetch_lists.bl_idname, text="Fetch")
+        row.prop(context.window_manager.package_manager_settings, "url")
+        row.operator(pkg_ops.PACKAGE_OT_fetch.bl_idname, text="Fetch")
 
 def register():
     bpy.utils.register_class(USERPREF_PT_packages)