adding two new editor groups, bridges and obstacles,

...and reordering most terrains in the editor to put common, simple
choices at the top.

data/core/editor/terrain-groups.cfg

@@ -67,6 +67,13 @@
     core=yes
 [/editor_group]
 
+[editor_group]
+    id=obstacle
+    name= _ "obstacle"
+    icon="group_obstacle"
+    core=yes
+[/editor_group]
+
 [editor_group]
     id=village
     name= _ "village"
@@ -81,6 +88,13 @@
     core=yes
 [/editor_group]
 
+[editor_group]
+    id=bridge
+    name= _ "bridge"
+    icon="group_bridge"
+    core=yes
+[/editor_group]
+
 [editor_group]
     id=special
     name= _ "special"