EI S13: Rewrite dragon's AI

Most part of the old AI had no evident effect: the dragon just
moved here and there without attacking you, and since he flied
over shallow water, it was impossible to chase him. The micro
ai makes Khrakhras wander around and attack you when you
approach his castle.

(cherry-picked from commit 64917b7)

data/campaigns/Eastern_Invasion/scenarios/13_The_Drowned_Plains.cfg

@@ -45,87 +45,6 @@
         user_team_name=_"Evil"
         gold=0
         no_leader=yes
-        [ai]
-            #don't target any of the default things
-            village_value=0
-            leader_value=0
-            [goal]
-                name=protect_unit
-                [criteria]
-                    side=3
-                    canrecruit=yes
-                [/criteria]
-                value=0
-            [/goal]
-            #stay in the middle of the map
-            [avoid]
-                [not]
-                    x=14-32
-                    y=10-45
-                [/not]
-            [/avoid]
-            #and target anyone in it (otherwise it will have nothing to do when
-            #it can't attack in the current turn)
-            [goal]
-                name=target
-                [criteria]
-                    #who knows what would happen if it targetted itself
-                    [not]
-                        type="Skeletal Dragon"
-                    [/not]
-                    x=14-32
-                    y=10-45
-                [/criteria]
-                value=100
-            [/goal]
-            #I don't know if this is needed, but it sure would be stupid to use
-            #any grouping behavior when there's only one unit on this side!
-            grouping=no
-            #not sure if this really does anything; it was copied from IP's code
-            aggression=1.0
-            leader_aggression=0.4
-            caution=0.0
-        [/ai]
-
-        #give the skeletal dragon something to do when there are no
-        #enemies available let it look like it's wandering around
-        #semi-randomly
-        [ai]
-            [leader_goal]
-                x,y=14,10
-            [/leader_goal]
-            time_of_day=dawn
-        [/ai]
-        [ai]
-            [leader_goal]
-                x,y=23,26
-            [/leader_goal]
-            time_of_day=morning
-        [/ai]
-        [ai]
-            [leader_goal]
-                x,y=23,26
-            [/leader_goal]
-            time_of_day=afternoon
-        [/ai]
-        [ai]
-            [leader_goal]
-                x,y=32,32
-            [/leader_goal]
-            time_of_day=dusk
-        [/ai]
-        [ai]
-            [leader_goal]
-                x,y=32,10
-            [/leader_goal]
-            time_of_day=first_watch
-        [/ai]
-        [ai]
-            [leader_goal]
-                x,y=14,32
-            [/leader_goal]
-            time_of_day=second_watch
-        [/ai]
     [/side]
 
     [event]
@@ -167,6 +86,20 @@
             canrecruit=yes
         [/unit]
 
+        [micro_ai]
+            side=3
+            ai_type=big_animals
+            action=add
+
+            [filter]
+                id="Khrakrahs"
+            [/filter]
+            [filter_location]
+                x=23-32
+                y=21-26
+            [/filter_location]
+        [/micro_ai]
+
         #buff the dragon with some magic resistances
         [object]
             name= _ "Null Stone"