FTrace: support timebase conversion (equivalent of trace-cmd's --ts2secs HZ option)

[fabrizioiannetti]

If a trace is recorded using a counter as clock, the scaling value to seconds must be given manually as it is not part of the trace (at least from what I could find only the clock name can be extracted), similarly to what one does with trace-cmd:

trace-cmd report --ts2secs <Hz>

Not sure how this could be implemented in the UI, maybe a an entry in the context menu along with Apply Time Offset..., e.g. Apply Time Conversion... ?

I'm willing to work on this as I have some interest in it.

Any suggestions/pointers are appreciated :-).