Support for receiver tuning

[Meisterschueler]

While configuring a receiver it is good if we could visualize the signal-to-noise-ratio, max. range and the 'rx beams' (like in glidertracker.de).
Ideas:

1. show the signal-to-noise-ratio of the sender instead (or additional) to the current competition sign. -> replace checkbox 'Show registration' with selection box label with options ['registration','sensitivity']
2. show the max. range (last x seconds) of the receivers instead (or additional) to the receiver name. -> add a selection box with labels ['name','max. range']. Max. range should consider last 60s (or less?)
3. show the beam from senders to receiver. -> additional checkbox for 'rx beam'