Add recording indication

[ryonakano]

Originally from #175 (comment):

If I can just pick your brain for a quick feature request, it would be nice to have some sort of indication of signal, maybe just a little red light when there is speech, something... because I just made a recording without my microphone plugged in and didn't even notice until I saw the file size :)

[nekohayo]

Perhaps you could take inspiration from the gnome-sound-recorder's realtime waveform visualization implementation?

[nekohayo]

Other alternatives include:

• Using GtkProgressbar or GtkLevelbar to indicate volume of the microphone, system, or both. This is what PulseCaster does.
• Imitating Pitivi's new vumeter widget implementation

[ryonakano]

Yeah, it would be ideal to make something similar to the indication seen in GNOME Sound Recorder; showing waves would be common in this situation.

[nekohayo]

The only thing is, I'm not entirely sure (yet) if the waves "as implemented in g-s-r" are the ones causing this CPU-eating performance issue on startup, but since your application is simpler and not trying to list all previous recordings, only show a live waveform while it's happening, then you will probably not have the same problem.