Async support: downloading, json parsing, and add-on install

Downloading, parsing index.json, and installing add-ons all now are handled asynchronously using the asyncio module. These operations will no longer block Blender, allowing them to run in the background.

package_manager/__init__.py

@@ -69,7 +69,18 @@ class PackageManagerPreferences(AddonPreferences):
     
     def draw(self, context):
         layout = self.layout
-        layout.operator("wm.update_index", text="Update List",  icon='FILE_REFRESH')
+        
+        split = layout.split(percentage=1.0/3)
+        if (len(self.pm_addons) == 0 or networking.download_install_status in
+            ("Processing response", "Downloading update",
+            "Processing failed", "Update failed")):
+            split.label(text=networking.download_install_status or
+                        "Update add-on list.")
+        else:
+            split.label(text="Available add-ons:")
+        split.separator()
+        split.operator("wm.update_index", text="Update List",  icon='FILE_REFRESH')
+        
         rows = 1 if len(self.pm_addons) == 0 else 4
         layout.template_list("UI_UL_list", "addons_list", self, "pm_addons",
             self, "pm_addons_index", rows=rows)
@@ -93,7 +104,14 @@ class PackageManagerPreferences(AddonPreferences):
         else:
             split.label(text="Installed: No")
         split.separator()
-        split.separator()
+        
+        if (networking.download_install_status not in
+            ("Processing response", "Downloading update",
+            "Processing failed", "Update failed")):
+            split.label(text=networking.download_install_status)
+        else:
+            split.separator()
+        
         split.operator("wm.addon_download_install",
                         text="Install from Web",
                         icon='URL').addon = addon.module_name