Maintain approx position in filtered window when changing filter regexp

[dunkpa]

This is a feature request.

I often parse very large log files, and would typically change the filter regexp a number of times when searching for/debugging a problem. Each time I do this, the filter window repositions so it's showing the earliest match, and I have to scroll down searching for my previous position, which can be quite hard to find (easier with 0.92 since I can search). It would be quite handy if the filter window repositioned to e.g. maintain the middle line number after updating the regexp.

[variar]

These commits address the issue to some extent 3bf7dbb, 43a7b9e, e1d543d.

I may try to prepare pull request for current master.

[nickbnf]

Yes please @variar , I'll merge that in!