GUI plugin for self tests

Detects which nodes are advertising a self_test service, makes it possible to run them, and display the results.

To test it, you can use any example from self_test package, for example:
 > rosrun self_test no_id_selftest
 > rosrun self_test error_selftest __name:="node_2"

Then start the GUI and select self_test plugin from Shadow Robot plugins.