Make some improvements to the pitch tracker widget

[celeste-sinead]

• adds a display of the current pitch and note to the right of the graph, which is nice for controlling pitch in real time. At this point #234 seems quite fully satisfied.
• shows minor grid lines in the frequency axis, instead of time, since that's usually what one wants more precision for in this graph
• replaces minimum SNR with minimum amplitude, which is kinda crude and needs tweaking to each environment, but once tweaked generally does a good job of avoiding detecting pitches in background noise (including pitched background sounds).

[tlecomte]

Thanks again @celeste-sinead !