libovcc/ovcc-board implement foreach with an iterator

We can't break HashTable foreach, so implement our foreach with a HashTableIter
instead of calling GLib's so that the table may be modified while iterating
and iteration stopped immediately.

libovcc/src/ovcc-board.vala

@@ -466,13 +466,18 @@ namespace OVCC
      */
     public bool @foreach (BoardForeachFunc f)
     {
-      bool keep_going = true;
-      this._board.foreach ((k, v) => {
-        if (keep_going) {
-          keep_going = f (this, k, v);
+      HashTableIter<Position?,Tile> iter = HashTableIter<Position?,Tile> (this._board);
+      Position? pos;
+      Tile tile;
+      bool went_through = true;
+
+      while (iter.next (out pos, out tile)) {
+        if (f (this, pos, tile) == false) {
+          went_through = false;
+          break;
         }
-      });
-      return keep_going;
+      }
+      return went_through;
     }
     
     /**