Fixed an occasional crash bug in rviz plugin caused by gui calls in non-gui thread.

It's not clear to me when exactly this happens, but it was happening enough to me that I fixed it.

The code looks like it should be OK, because the subscription is done via "update_nh_", which should use the default global callback-queue.  However, I tried printing out the thread ID in the callback function and the GUI code, and sometimes the message callback happened in a different thread.

The fix was easy, just use a signal/slot call instead of a direct method call.