Notify entry/group parent on deletion.

Also make the root group pseudo static, i.e. it shouldn't be changed
after the database has been fully constructed.

src/core/Database.h

@@ -53,6 +53,10 @@ public:
 
     /**
      * Sets group as the root group and takes ownership of it.
+     * Warning: Be careful when calling this method as it doesn't
+     *          emit any notifications so e.g. models aren't updated.
+     *          The caller is responsible for cleaning up the pervious
+                root group.
      */
     void setRootGroup(Group* group);