ovccclient-client: trigger place_pawn() when Idle

Like for adding tile, place a pawn when Idle (otherwise all actions are chained
since signals are synchronous).

libovccclient/src/ovccclient-client.vala

@@ -171,7 +171,10 @@ namespace OVCCClient
 
       sigqueue.add (game.board, game.board.tile_added.connect ((t, p) => {
         if (game.current_player == player) {
-          this.place_pawn (p);
+          Idle.add (() => {
+            this.place_pawn (p);
+            return false;
+          });
         }
       }));