Make completion window move to top when cursor is below half

[vv9k]

Closes: #307

[archseer]

Neat! Did you test that it works correctly with splits? A vertical split for example

[vv9k]

Vertical does seem to work, but with horizontal it is still displayed on the bottom. I will push a fix soon.

Edit: Actually rethinking it now, it might not be as trivial to determine whether the cursor is below half of screen. I will have to dig some more.

[vv9k]

Now it correctly displays even with splits.

Edit: I still found a split configuration where it doesn't work, so this needs an update

[archseer]

Call it either area or viewport to match terminology elsewhere

[vv9k]

Right, done.

[vv9k]

I'm pretty sure it works as expected now, done tests with multiple split confiurations and it is correctly either displayed on top or bottom so that the cursor is always visible.