created new icons to differentiate mainline, but non-core terrain...

...groups and UMC terrain groups.  UMC authors have 3 styles to choose
between to keep things from getting too confusing.  Also created some
group buttons for future use.

data/campaigns/Delfadors_Memoirs/_main.cfg

@@ -97,7 +97,7 @@
 [editor_group]
     id=dm
     name= _ "Delfador’s Memoirs"
-    icon="group_custom"
+    icon="group_mainline"
 [/editor_group]
 {campaigns/Delfadors_Memoirs/terrain.cfg}
 {campaigns/Delfadors_Memoirs/terrain-graphics/_final.cfg}

data/campaigns/Descent_Into_Darkness/_main.cfg

@@ -72,7 +72,7 @@
 [editor_group]
     id=did
     name= _ "Descent into Darkness"
-    icon="group_custom"
+    icon="group_mainline"
 [/editor_group]
 {campaigns/Descent_Into_Darkness/utils/terrain.cfg}
 #endif

data/campaigns/Under_the_Burning_Suns/_main.cfg

@@ -154,7 +154,7 @@
 [editor_group]
     id=utbs
     name= _ "Under the Burning Suns"
-    icon="group_custom"
+    icon="group_mainline"
 [/editor_group]
 {campaigns/Under_the_Burning_Suns/utils/terrain.cfg}
 {campaigns/Under_the_Burning_Suns/utils/terrain_graphics.cfg}