Move instarepl widgets with other widgets in :widgets

* Moved widgets from ::widgets to :widgets in editor
* This fixes clearing instarepl widgets via command in LightTable/LightTable#1324
* This may also fix ::cleanup-on-destroy which was looking in :widgets

@ibdknox While deving on this I noticed that any widgets that are cleared are never removed from editor's :widgets. They just hang around as (atom nil). Possibly a source of another bug?