EXPERIMENTAL_SCRUBBING_BASIC enables middle-click-drag to scrub, or to seek
when shift is down.
EXPERIMENTAL_SCRUBBING_SMOOTH_SCROLL adds middle-double-click-drag to
scrub or (with SHIFT down) to seek while keeping the playhead at the midline
of the track display.
EXPERIMENTAL_SCRUBBING_SCROLL_WHEEL allows control wheel to vary the speed
limit, while scrubbing (not seeking).
All three are enabled in this commit.
Also notice the changes to status bar messages and the Mouse Preferences dialog.
Scrubbing works only in Select and Multi tools.
ESC Key toggles snapping of center frequency to peaks (no visual indication of snap to yet)
Description added to mouse prefs - fix later.
Shift now required to keep top/bottom frequency pinned whilst (re)dragging the other one.
Code for status bar messages simplified and made more consistent, and no longer hard codes assumption that preferences is Ctrl-P when prompting user about it.
This relies on three new nyquist scripts to actually do the editing. The peak-snapping code in FrequencyWindow has been extracted into a new class, SpectrumAnalyst, to provide peak-snapping in spectrogram too.
