Improvements for RQT Free Gait

[pfankhauser]

• [x] Maybe rename from rqt_free_gait to rqt_free_gait_monitor?
• [x] Remove name information.
• [x] Update feedback output based on new feedback information.
• [x] Don't use pre-programmed limb names (LF_LEG etc.) and just use the given strings instead.
• [x] Add pause and stop execution service call with service name from parameter free_gait/pause_execution_service and free_gait/stop_execution_service