pin selection of closest entity in map editor

Pressing the middle mouse button toggles a pin on the "closest entity"
selector. The HUD will indicate the lock by showing "pinned" instead of
"closest" entity. All actions, that would otherwise affect the closest
entity, will affect the pinned entity instead. Deleting the locked entity
will unlock it.

toggleclosestentpin
  Toggles the pin on the "closest ent" selector. Bound to MOUSE3
  by default.

DOCREF togglelockclosestent

source/src/editing.cpp

@@ -106,7 +106,7 @@ char *editinfo()
     {
         entity &c = ents[e];
         int t = c.type < MAXENTTYPES ? c.type : 0;
-        formatstring(info)("closest entity: %s (%s)", entnames[t], formatentityattributes(c, true));
+        formatstring(info)("%s entity: %s (%s)", pinnedclosestent ? "\fs\f3pinned\fr" : "closest", entnames[t], formatentityattributes(c, true));
         const char *slotinfo = "unassigned slot";
         if(t == MAPMODEL)
         {