Merged addon-packaging branch.

You can now install and uninstall special add-on packages (.nvda-addon files) for NVDA that provide custom app modules, global plugins, braille display drivers and synth drivers. The Manage Add-ons menu item can be found under Tools in the NVDA menu. The User guide provides more info about the Add-ons manager, and the developer guide provides info on how to create these packages for distribution.

diff --cc source/gui/__init__.py
Simple merge

diff --cc source/gui/installerGui.py
Simple merge

diff --cc user_docs/en/changes.t2t
Simple merge