Allow oasis to be placed on any terrain - alias to base terrain

changelog

@@ -31,6 +31,8 @@ Version 1.9.9+svn:
    * The side metatable provides also the side number
      (like wesnoth.sides[i].side, which would be i)
    * add function wesnoth.debug
+ * Terrain:
+   * Oasis may be placed on any terrain, aliased to base
  * Unit changes and Balancing
    * All mounted units now have forest defines capped at 30%. This reduces their
      defense on forested hills from 40% to 30%

data/core/terrain.cfg

@@ -299,8 +299,9 @@
     symbol_image=sand/desert-oasis
     id=oasis
     name= _ "Oasis"
-    string=Dd^Do
-    aliasof=Ds
+    default_base=Dd
+    string=^Do
+    aliasof=_bas
     heals=8
     #gives_income=true
     editor_group=desert, forest

players_changelog

@@ -30,6 +30,9 @@ Version 1.9.9+svn:
      Italian, Japanese, Korean, Latin, Lithuanian, Old English,
      Portuguese (Brazil), Russian, Slovak, Swedish.
 
+ * Terrain:
+   * Oasis may be placed on any terrain, aliased to base.
+
  * Unit changes and Balancing
    * All mounted units now have forest defense capped at 30%. This reduces their
      defense on forested hills from 40% to 30%