SigMF annotations edition, signal measurements/computations and burst detection

[ng88]

Summary

This PR brings in the features developed in my fork: SigMF
annotation editing, per-selection signal measurements, automatic burst
detection and PNG export.

I'm opening this as a single PR to show the whole picture, but I'm happy to
split it into separate, smaller PRs. Just let me know what you'd prefer.

What's included

SigMF annotations
Annotations from the SigMF metadata file can be created, edited and
deleted directly from the spectrogram, and written back to the .sigmf-meta
file.

Optional center frequency
When set, the frequency axis shows absolute frequencies instead of baseband
offsets.

Measurements on a selection
Selecting a range of samples can compute: symbol rate, center frequency,
power/RMS, autocorrelation period, occupied bandwidth, and a modulation
classification guess (OOK/ASK, BPSK/QPSK/8PSK, 2FSK/4FSK/GMSK, OFDM).

Burst detection
Automatic detection of bursts, listed in a side panel; clicking an entry jumps
to that burst in the view.

PNG export
Save plots as an image.

Notes for review

• No new dependencies
• The modulation classifier is heuristic and best-effort