Watcher improvements

Now uses GTree instead of GPtrArray making things somewhat less exponential so we can now nicely handle lots of terminals

When running in the background watcher runs at a lower frequency than when a window if focused, unfocused windows now send a notification when a child completes

Combined performance boost and slower cycles mean KGX should now idle at 0-1% cpu when in the background

Data from the watcher is now used to provide a "Are you sure" dialog when closing a terminal with active children