Add option to re-write header timestamp

[awesomebytes]

In some cases rewriting the timestamp of a rosbag eases debugging (so you can play the bag in a loop), specially with the /tf topic. Also if you are mixing real data of a robot with a rosbag.

It would be nice to have an option to rewrite the timestamps of a specific topic. The first idea that comes to my mind would be having another option on right click to be Publish rewriting Timestamps (probably something shorter). I don't know if this is feasible to be added as a plugin, if so, and if anyone else thinks this may be useful, I'll try to implement it.

Maybe would be nice to have rosbag play have the same capability.

[rmaronson]

+1 on this, but would be ideal as part of rosbag as well/instead of rqt_bag. Didn't want to duplicate issues unnecessarily!