DM S20: Improve leveling of units and give player a note about it

Fixes #4219

data/campaigns/Delfadors_Memoirs/scenarios/20_Prince_of_Wesnoth.cfg

@@ -137,6 +137,10 @@
                 bonus=yes
                 carryover_percentage=80
             [/gold_carryover]
+
+            [note]
+                description=_ "As a generation has passed, all your elvish units have gained a level and some experience."
+            [/note]
         [/objectives]
     [/event]
 

data/campaigns/Delfadors_Memoirs/utils/sides.cfg

@@ -535,12 +535,24 @@
             race=elf
         [/not]
     [/kill]
-    [modify_unit]
-        [filter]
-            side=1
-        [/filter]
-        type=""
-    [/modify_unit]
+    [lua]
+        code=<<
+            local helper = wesnoth.require "helper"
+            local units = wesnoth.get_units( { side = 1 } )
+            for i = 1, #units do
+                -- pick an advancement randomly, and remove others so advance() doesn't show the dialog
+                if units[i].advances_to[1] ~= nil then
+                    local r = wesnoth.random ( 1, #units[i].advances_to )
+                    local t = { }; t[1] = units[i].advances_to[r]
+                    units[i].advances_to = t
+                end
+                -- level the unit and give it a random amount of experience up to half of max
+                units[i].experience = units[i].max_experience
+                units[i]:advance( )
+                units[i].experience = wesnoth.random ( 0, helper.round( units[i].max_experience / 2 ) )
+            end
+        >>
+    [/lua]
     [heal_unit]
         [filter]
             side=1
@@ -551,15 +563,12 @@
         restore_attacks=yes
         restore_statuses=yes
     [/heal_unit]
-    [modify_unit]
-        [filter]
-            side=1
-            [not]
-                id=Kalenz,Chantal
-            [/not]
-        [/filter]
-        x,y=recall,recall
-    [/modify_unit]
+    [put_to_recall_list]
+        side=1
+        [not]
+            id=Kalenz,Chantal
+        [/not]
+    [/put_to_recall_list]
     #Lowel and Eldred are the enemy humans when this fires
     [kill]
         race=human