Remove current_player= setting and add save_id=human_player

(fix for bug #19188)

This causes statistics, scenario overwiew and turn dialog to show the
save_id instead of current_player. This doesn't break stringfreeze
since save_id isn't translatable. Any bug report complaints about
these dialogs no longer showing player name's translation shall be
rejected. >:|

Uninvasive workaround. Adding a dummy side would require many changes
and complicate the wml code. I've been using save_id=human player in
my addon campaign which involves leader change as well sine ever
without anyone complaining so hopefully...

data/campaigns/tutorial/scenarios/1_Tutorial.cfg

@@ -81,7 +81,9 @@ border_size=1
         id=student
         name= _"Konrad"
         profile=portraits/konrad.png
-        current_player= _"Konrad"
+        save_id=human_player
+        persistent=yes
+        gold=100
         canrecruit=yes
         side=1
         controller=human

data/campaigns/tutorial/scenarios/2_Tutorial.cfg

@@ -354,7 +354,8 @@
         type=Fighter
         id=student
         name= _"Konrad"
-        current_player= _"Konrad"
+        save_id=human_player
+        persistent=yes
         canrecruit=yes
         side=1
         gold=123