joint_state_publisher: fixed crash when source_cb directly updates sliders

When the JSP updates the wx sliders directly from the ros subscriber callback, it crashes reproducibly. Going through a wx event to update the sliders fixes this,
Errors that were produced by the old version range from assert failures to stack corruption...:
(python:18864): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

*** glibc detected *** /usr/bin/python: double free or corruption (fasttop): 0x00007f92ac01b390 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x7eb96)[0x7f92cbf11b96]
...