Added WidgetList class and explicit widget definition, and started using it to build UI
Removed JoinDescriptor (GGRC.Mappings now does everything we needed from it), and removed menu_treefrom dashboard controller
Merge remote-tracking branch 'origin/feature/refactor-widget-creation' into feature/refactor-widget-creation

Conflicts:
	src/ggrc/assets/javascripts/apps/business_objects.js
	src/ggrc/assets/javascripts/apps/dashboard.js
	src/ggrc/assets/javascripts/apps/quick_search.js
	src/ggrc/assets/javascripts/models/mappings.js