Refactored plugin architecture out of WebThing.cs

Added support for adding plugins via configuration files, obsoleting
'plugins.conf'.  Moved plugin loading out of WebThing and into
PluginManager.  Slight modification to WebThing launch script exports
WEBTHING_HOME, which is used in PluginManager to find the plugin
directory.  Minor modifications to WebThing widget attachment names;
Vimish and LoadProgress changed to suit.