GITFNS prc menu is added to the screen instead of just popping up (#1695)

So the full set of available PRs (with their draft/approved status)
remains visible. You can poke from one to the other. May be useful when
we go through the PRs in Monday meetings. (Still doesn't bother with a
menu at all if there is only one PR to work on.)

On this first pass, the menu is placed (but not attached) just under the
current exec window, on the hope that this keeps it out of the way of
the windows that are created during the comparison.

Try it, let me know how this feels, whether this is an improvement over
the previous pop-up.

Author: MattHeffron