add option to move tracks

It could be usefull to provide a way to move tracks from GpxPod interface. When file names are not very expressive, it is slow to check the file content in GpxPod and then to go to "Files" app to move the file.

This feature could be a button on top of the track table named "move selected track to" which would ask where to put selected tracks and perform the file move inside GpxPod.