qt: adjust advanced prefs tree filtering behaviour

The previous behaviour upon filter change was that:
 - If the search string was not empty then everything visible was expanded.
 - If empty, then everything would be collapsed except the selected node.

This is rather annoying IMO, for instance - from a default starting state
where top-level nodes are expanded and 2nd-level collapsed, if you apply a
filter and then subsequently remove it, you annoyingly end up with more
collapsed than you started with.

I feel that it works much better to just leave the expanded/collapsed state
untouched, preserving the initial state and any changes the user has made
before/after filtering, even though this forces users to expand any
2nd-level filter results they are interested in for themselves. It's also
much more simple.