Add ability to manipulate bag files (trim, select subset of topics)

[llvs]

To make clear: This is more of a feature request than an issue.

It would be very handy to have the ability to graphically select a subset of topics and a time span and then to be able to save such a selection as a new bag file.
This functionality can be mimicked by replaying a bag file and performing a rosbag record with the necessary parameters, but this is cumbersome and not as clean as stripping unneeded parts from a rosbag.

Best regards