rqt_bag: timestamp printing issue

[ablasdel]

rxbag used to print raw timestamp incorrectly so it still does:

In the timeline status bar, rxbag incorrectly prints the nanosecond portion of the raw timestamp. The code currently does this:
'%d.%s' % (self.timeline.playhead.secs, str(self.timeline.playhead.nsecs)[:3])

which will take the 3 highest digits of nanoseconds and print them like a decimal. This is meaningless. A better solution is attached that does this:
'%.3fs' % self.timeline.playhead.to_sec()

which is also used in the same function for printing the elapsed time.

If you were looking at the raw timestamp, it could be non-monotonic because of this bug, falsely indicating that certain topics arrived after other topics even though the visualization is correct.

For more information see the trac ticket.

migration of of trac ticket:
https://code.ros.org/trac/ros/ticket/3824