Add centre frequency and span indicators to waterfall plot on the station console

[tstranex]

In the online console Waterfall plot, add center frequency and span indicators.

The tricky part is that the frontend doesn't know sample rate that carpsd is using until the IQ data is uploaded.

Possible solution:
Extend the carpsd ReceiverGetState call to return the sample rate.
Then render the indicators on the canvas using javascript.

Relevant code:
GetStateDict in carpsd/carpsd/receiver.py (and implement in fcd_receiver.py and rtlsdr_receiver.py)
static/receiver.js