... SpectrogramSettings does that instead, and Preferences or View Settings
are the user interface for changing it.
Handle invalidation of spectrogram pixel cache for scale type changes,
just as for other changes of settings. No more
TrackArtist::InvalidateSpectrumCache().
View type of track now switches to Spectrum when applying or OKing the
View Settings... dialog and the Spectrogram page is open (and for now
it is still the only page)
... writes global preferences explicitly, and only when it is the default
settings object.
Also impose validation of settings when constructing from preferences.
Don't read "/Spectrum/" preferences anywhere but in SpectrumPrefs.cpp.
Don't store global values in TrackArtist.
Possibly some day, there will be other instances of SpectrogramSettings so we
can have independent per-track settings.
