Disable vertical text centring

[lproven]

I find the way the app tries to keep the cursor in the middle of the screen distracting. I prefer the more conventional behaviour of scrolling only when the cursor is going to go off-screen.

I suggest the ScrollLock key for this, but many distros disable it. I don't know if the app could trap it just when it has focus.

If not, a preference in Global Settings would do.

[lproven]

Overlaps with #89

[davidgiven]

Duplicate of #89