Move preloading of widget channels from widget_starter to widget

[ovidiuch]

The idea is to encapsulated the waiting for channels to initialize (because of async requires of models and collections) inside the widget. It currently is in the widget starter.

This will also allow us to subscribe to newly created channels all inside a widget's initialize method. This is not currently possible.

This is a ballpark schema of the widget_starter > widget -> datasource flow