Rewrite the QIviSearchAndBrowseModel backend interface

The QIviSearchAndBrowseModel backend interface currently doesn't work as smoothly as intended.

• Split the backend into multiple classes
• The backend should have a way to decide which signals are intended for which frotenend, or whether they are intended for all frontends
• The backend needs a way to know when a new frontend got added and also when it gets removed, to manage the local cache and free data.