Added Gazebo activity and service

I added a new package rtt_gazebo_activity that provides a new activity that is run by a GazeboActivityManager singleton instance for each gazebo update.

Use cases:
1. If RTT is running within gazebo itself using the rtt_gazebo_plugin, the GazeboActivity Manager is triggered by Gazebo's update event within the gazebo thread.
2. If RTT is running in a standalone thread, the new gazebo service will subscribe the `/clock` topic, manipulate the TimeService and trigger the GazeboActivities.

Furthermore the gazebo service assigns a new GazeboActivity to its owner when it is loaded into a TaskContext directly. I did not find a way to use the `gazebo.setGazeboActivity(peer)` operation directly from the deployer or from scripting, so this is more a workaround than a nice solution.

I personally use only the second use case until now, so the first would need some testing before merging.