ensureCursorVisible() should vertically center the cursor in the view

I'd say the behavior of ensureCursorVisible() is not very helpful; from the user's perspective, it unpredictably puts the cursor at either the very top or the very bottom of the visible area, and the user can't really guess which, so they then have to do a little visual hunt for where the cursor has ended up. This is particularly annoying if the cursor is just an insertion point. I'd suggest that it should put the cursor at the vertical center of the view, not the top or bottom, when possible, making it more predictable, easier to find visually, and also more useful to the user, since the user will typically want to see the cursor in the context of the surrounding lines above and below.

This bug was split out from QTBUG-83105, by request.