Use common layout for widgets
Install broccoli-static-compiler & broccoli-merge-trees
Move base widget under a widgets/system
Don’t override the widget’s layout with a wrong one
No need to confirm widget removal right now
Dismiss insert-widget after performing an insert
Move deserialization to widget base class
Simplify widget handle HTML with widget-handle-button
Comment out app header until there is some functionality
Simplify insert-widget-popover HTML
Move widget-handle into base layout