Ability to get a full map/overview of all entities and properties on the page

[wimleers]

Depending on how the UX is designed to work, it seems it could be necessary to have a full map/overview of all entities and their properties on the page. And more specifically, for the respective EditableEntity widgets and PropertyEditor widgets.

E.g. in the case of Edit, the app controller must be able to set the state inactive on every PropertyEditor except on the one that is now activating (i.e.: "remove all decorations from all things that can be edited on this page, so that everything looks identical to the regular page, plus a PropertyEditor widget for the property you're currently editing").

/cc @frega