Fix display of child and parent records in schema browser

Clicking on the table of query results in the schema browser more than once causes Eclipse to hang (on a Mac). The underlying code is fairly convoluted so rather than trying to diagnose and fix it I have replaced it with a simpler approach of doing the work in a mouse listener and this works reliably.

The icon display logic - that looks like it had something to do with displaying checkbox icons - just resulted in a red "icon missing" red square being displayed so I've removed that too.