Emit row-has-child-toggled when a first node becomes visible in a level

Emit row-has-child-toggled when a first node becomes visible in a level

If we have a level with zero visible nodes and the first node becomes
visible in that level, then parent has just become a "real" parent node.
In such a case we need to emit row-has-child-toggled.  This only applies
to non-root levels that have a parent.  This problem was also found when
writing the unit test, the respective cases in the unit test have been
corrected.

This fixes bugs:
 Bug 372010 - Filtering not working properly
 Bug 525965 - Filtered and sorted GtkTreeView is missing rows