Expose register method directly in AddonManager

By doing so the actual registry returned by getRegistry does not need to be the full fledged
CombiningRegistry implementation. A proxied object is returned, which only provides the
AddonRegistry interface. When sandboxing is activated, only the register method must be protected,
not the whole registry.
Change the addon search path from $OEC_HOME/plugins to $OEC_HOME/addons
Rename functions in AddonRegistry

listPlugins -> listAddons
downloadPlugin -> downloadAddon
Prevent leaks of actual registry types in AddonManager
Fix FacadeInvocationHandler
Fix config entries in add-on manifest
Fix small bugs in add-on GUI