Completion list positioning enhancements

[kindermannhubert]

Now we allocate an empty space under the current line for displaying the completion list and its items descriptions.

We could solve this space wasting by dynamically moving the completion list upwards when it does not have space below the current line. When we have enough space below the behavior would be the same as today.
So it would look something like this:

Possibly it could be configurable but not sure if someone would want to fall back to old behavior.

[kindermannhubert]

Not sure if this is possible to do in some reasonable way. We would need to render over already existing output and the question is how to restore it back.

There exists alternate buffer but we would need it to contain everything from the main buffer. From what I tested alternate buffer is empty and I don't know if it's possible to get there contents of the main buffer.

On Windows, it could probably be done with custom screen buffers - it should be possible to copy content between them.
Elsewhere, I don't know.

Maybe @waf has some other idea?