qtgui: Add feature to offset the waterfall, to keep a carrier at the same X position as tuning changes

[ThomasHabets]

One option is to offset this using a variable, e.g. set_offset(), but it has the drawback of not being in sync with actual data coming in.

So maybe better is to allow the offset to be set via tags. E.g. as a tag with a frequency is sent from SDR hardware, the waterfall can adjuts.

This will make it easier to see carriers as one tunes around.