New "mechanical" trait for mechanical unit.

This is only meant to display those units are immune to plague, poison and drain

mention the drain and plague immunity in the "undead" trait description.

changelog

@@ -13,6 +13,8 @@ Version 1.3.4+svn:
    * revised maps: Blitz, Cynsaun Battlefield, Hamlets, Sablestone Delta,
      Silverhead Crossing, Sulla's Ruins, Clash
    * fix MP crash on next scenario, thanks to Rhuvaen
+ * units
+   * new "mechanical" trait, meant to show mechanical unit are immune to poison
  * user interface:
    * allow unfocused widgets to steal the focus instead of just borrowing it
    * frequency of idle animations halved.

data/core/help.cfg

@@ -20,7 +20,7 @@
     [section]
         id=traits_section
         title= _ "Traits"
-        topics=traits,traits_fearless,traits_intelligent,traits_resilient,traits_strong,traits_quick,traits_dextrous,traits_healthy,traits_loyal,traits_undead
+        topics=traits,traits_fearless,traits_intelligent,traits_resilient,traits_strong,traits_quick,traits_dextrous,traits_healthy,traits_loyal,traits_undead,traits_mechanical
         # Not used since generated text was not really suitable.
         #generator=traits
         sort_topics=no
@@ -326,6 +326,14 @@ During campaigns, certain units may opt to join the player's forces of their own
 Undead units generally have 'Undead' as their only trait. Since Undead units are the bodies of the dead, risen to fight again, poison has no effect upon them. This can make them invaluable in dealing with foes who use poison in conjunction with their attacks."
     [/topic]
 
+    [topic]
+        id=traits_mechanical
+        title= _"trait^Mechanical"
+        text= _"Mechanical units are immune to poison, also drain and plague doesn't work on them." + _"
+
+Mechanical units generally have 'Mechanical' as their only trait. Since mechanical units don't really have life, drain, poison and plague has no effect upon them."
+    [/topic]
+
     [topic]
         id=traits_dextrous
         title= _"Dextrous"

data/core/macros/traits.cfg

@@ -16,12 +16,22 @@
 #enddef
 
 #define TRAIT_UNDEAD
-    # Units with trait Undead cannot be poisoned.
+    # Units with trait Undead cannot be drained, poisoned or plagued.
     [trait]
         id=undead
         male_name= _ "undead"
         female_name= _ "female^undead"
-        description= _ "Immune to poison"
+        description= _ "Immune to drain, poison and plague"
+    [/trait]
+#enddef
+
+#define TRAIT_MECHANICAL
+    # Units with trait Mechanical cannot be drained, poisoned or plagued.
+    [trait]
+        id=mechanical
+        male_name= _ "mechanical"
+        female_name= _ "female^mechanical"
+        description= _ "Immune to drain, poison and plague"
     [/trait]
 #enddef
 

data/core/units.cfg

@@ -132,8 +132,10 @@
     [race]
         name=mechanical
         not_living=yes
-        num_traits=0
+        num_traits=1
         undead_variation=null
+        ignore_global_traits=yes
+        {TRAIT_MECHANICAL}
     [/race]
 
     #       *   M O V E   T Y P E S   *