Add Unwalkable Overlay terrain (^Qov) (patch #3039)

I made the icon myself based on the Impassable Overlay terrain's. The
font is DejaVu Serif at size 35 with full grayscale hinting, which
should be the same as the Impassable Overlay icon's based on a visual
comparison (with very negligible rendering differences).

changelog

@@ -7,6 +7,8 @@ Version 1.11.0+svn:
      * Keep a spawned unit from disappearing on reload in scenario 6b
  * Language and i18n:
    * Updated translations: British English, Croatian, French, Italian
+ * Terrains:
+   * Added Unwalkable Overlay terrain (^Qov)
  * User interface:
    * Healing animations are now played when poison is cured.
    * The recruit and recall commands are restored when right-clicking on a

data/core/terrain-graphics.cfg

@@ -47,6 +47,7 @@
 
 # Editor overlays - for overlays that should only be visible in the editor
 {EDITOR_OVERLAY *^Xo  impassable-editor}
+{EDITOR_OVERLAY *^Qov unwalkable-editor}
 {EDITOR_OVERLAY *^Vov village/village-overlay-editor}
 {EDITOR_OVERLAY *^Cov castle/castle-overlay-editor}
 {EDITOR_OVERLAY *^Kov castle/keep-overlay-editor}

data/core/terrain.cfg

@@ -1151,6 +1151,17 @@
     editor_group=special
 [/terrain_type]
 
+[terrain_type]
+    symbol_image=void/void
+    editor_image=unwalkable-editor
+    id=unwalkable_overlay
+    editor_name= _ "Unwalkable Overlay"
+    string=^Qov
+    aliasof=_bas,Qt
+    mvt_alias=Qt
+    editor_group=special
+[/terrain_type]
+
 [terrain_type]
     symbol_image=void/void
     editor_image=void/void-editor