Tweak and add some more Xom tension spells

Battlesphere and Intoxication have been adjusted towards their new spell
levels, while two new Forgecraft spells have been introduced for the sake
of both variety and tactical screwery: Alistair's Walking Alembic feels
both appropriate for Xom's potion effect focus as well as leaving around
poison clouds to screw with a non-poison-resistant player's movement, while
Diamond Sawblades encourages repositioning mid-combat to be able to get any
value out of their free but arbitrary rare placement.

crawl-ref/source/xom.cc

@@ -98,26 +98,29 @@ static bool _action_is_bad(xom_event_type action)
 
 // Spells to be cast at tension > 0, i.e. usually in battle situations.
 // Spells later in the list require higher severity to have a chance of being
-// selected.
+// selected. Spells are sorted by level, then very roughly by power when
+// neither planned for nor capability to repeatedly cast it.
 static const vector<spell_type> _xom_random_spells =
 {
     SPELL_SUMMON_SMALL_MAMMAL,
     SPELL_DAZZLING_FLASH,
     SPELL_FUGUE_OF_THE_FALLEN,
     SPELL_OLGREBS_TOXIC_RADIANCE,
+    SPELL_BATTLESPHERE,
     SPELL_AWAKEN_ARMOUR,
     SPELL_MARTYRS_KNELL,
     SPELL_LEDAS_LIQUEFACTION,
     SPELL_FORGE_BLAZEHEART_GOLEM,
-    SPELL_BATTLESPHERE,
-    SPELL_INTOXICATE,
     SPELL_ANIMATE_DEAD,
+    SPELL_INTOXICATE,
     SPELL_SUMMON_MANA_VIPER,
+    SPELL_WALKING_ALEMBIC,
     SPELL_SUMMON_CACTUS,
     SPELL_DISPERSAL,
     SPELL_DEATH_CHANNEL,
     SPELL_SUMMON_HYDRA,
     SPELL_MONSTROUS_MENAGERIE,
+    SPELL_DIAMOND_SAWBLADES,
     SPELL_MALIGN_GATEWAY,
     SPELL_DISCORD,
     SPELL_DISJUNCTION,