Plugin is now added to the viewer using an inplace add on the viewer instead of on initialization of the plugin. This change means that operations requiring the viewer must be delayed until attach operation.
The `ptype` parameter of widget can now be set to 'plugin'. When this is the case, the plugin will set a plugin attribute whenever the widget is updated.
As an example, this commit adds a ComboBox widget which is hooked into the overlay color of the OverlayPlugin.
Tony S Yu