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

__init__.py

@@ -9,11 +9,18 @@ import bpy
 class PackageSettings(bpy.types.PropertyGroup):
     url = bpy.props.StringProperty(name="URL")
 
+# class PackageManager:
+    # # For some reason accessing 'settings' PointerProperty via wm.package_manager.settings gives a value error
+    # # but accessing it when not stored in this class (wm.package_manager_settings) is fine?
+    # # settings = bpy.props.PointerProperty(type=PackageSettings)
+    # pipes = []
+
 def register():
     from . import (pkg_ops, pkg_ui)
 
     bpy.utils.register_class(PackageSettings)
-    bpy.types.WindowManager.PackageManagerSettings = bpy.props.PointerProperty(type=PackageSettings)
+    # bpy.types.WindowManager.package_manager = PackageManager()
+    bpy.types.WindowManager.package_manager_settings = bpy.props.PointerProperty(type=PackageSettings)
     pkg_ops.register()
     pkg_ui.register()
 
@@ -22,5 +29,6 @@ def unregister():
 
     pkg_ops.unregister()
     pkg_ui.unregister();
-    del bpy.types.WindowManager.PackageManagerSettings
+    bpy.utils.unregister_class(PackageSettings)
+    del bpy.types.WindowManager.package_manager