Shortcut some tests in Model initialization

When Model objects (Albums and Items) are loaded from the database a lot of time is wasted checking if the object is dirty and if fields are fixed or flexattr.

This change alow bypassing these checks when we know that the data is correct ; it should only be used when loading Model from the database.

With this, listing all tracks in my test database (8200 items) takes 4 seconds while it took 31 seconds previously.