Search matches should scroll to the middle of the screen when possible

## Problem
Pressing `g` to locate the next match only scrolls the window a minimum amount to get match on screen - usually the bottom line. It would be more convenient to scroll it to nearer the middle of the screen where the eye naturally looks and would allow sight of all the context.

## Proposal
Use the `within_margin` parameter to scroll nearer the centre of the screen

## Prior Art

Geany does this.