Add "entity" for webui filtering

In the UI it can be good to have different "group of things" that can be good to display and aggregate.


Some will want to show some geographic dispatch, other want a technical view, other business elements.


So with the entities, we can have :

* realm and poller_tag : geographic separation

* hostgroups/servicegroups : technical ones

* template : technical one too


So in a dashboard, the user can choose between "entities" to show and agregate (worse state, importance, etc).


Idea No. 76