Addons UI (Basic)

Displays a list of custom UIs and auto loads custom UIs marked in
module-list.js to be auto loaded.  Allows custom UIs to be manually
loaded.
Added base module-list.js (for Addons UI)

Added base module-list.js which lists the core UIs (those shipped with
the game) and addons UI.
Merge pull request #6 from Mehuge/upstream-feature-addons

Basic Addons UI