3.0 widgets list from config file

Here is fix for possibility to load widgets array from config file. 
But maybe it is reasonable to use InstanceConfigTrait and rewrite code for it. For example method WidgetRegistry::clear cleanup all widgets. It can broke view totally.
If WidgetRegistry will use InstanceConfigTrait it is always possible to use default widgets list after clenaup.
We can be specific here. `string|array` instead of `mixed.