Freeciv

In #1375, there have been two ways proposed to give player the way to decide where an obteined superspec goes:

1. Add to city a unit that becomes superspecialist.
2. Settle a specialist from player's specialist hand.

Probably, we can make both actions, one unit-performed and the another player-performed, but both need new subtarget kind ASTK_SPECIALIST. My original idea though was defining the specialist in unit type properties and use existing "Join city" action, but with action subtarget control we may add alternative specialists by the same "great person" unit and this just looks better.

For the addition from hand, I will probably need help with interface part, at very least other than Gtk*. Maybe they would look in each possible target city like transparent specialist sprites, or risen above the common line with an arrow below, and definitely not partially shown; you click them to settle.