Allow to load a bag file without appending to the current timeline

[VictorLamoine]

If you want to load one bag then an other but don't end with with the two bag files in the same timeline right now the only option is to close rqt_bag and open the second bag file.

It would be nice to be able to reset the timeline and load the bag as the only one in the timeline.
But we need to keep the possibility to load and append a bag file.