- Use gcc-12 for Leap 15.6
- Update to 5.0
* Port to Qt6. The code now builds only with Qt6, and Qt5 is no
longer supported
* Add mechanism to ignore plugins on subsequent runs if they fail to
load, and to review ignored plugins from the preferences dialog
* Add Mel scale to the options for vertical spectrogram scale

* Add an opaque toggle to the spectrogram; switching it off gives
spectrograms some transparency, so they can be overlaid over other
layers, including other spectrograms

* Update opaque toggle in colour 3d layers so that it works like the
new one in the spectrogram. Previously colour 3d plot layers could
be translucent at low resolutions but not at higher ones; now the
setting affects all resolutions. Correspondingly make it default to
off, as in the spectrogram
* Add smoothing toggle directly to spectrogram parameters, like the
one already found in colour 3d plot layers. Previously spectrogram
smoothing could be toggled only from the preferences
* Stop snapping spectrogram frequency range to bin frequencies. This
already-confusing feature caused problems with the new support for
translucent spectrograms, which permits overlaying spectrograms of
differing bin counts that would be too hard to line up properly if
the frequency extents were always snapped to the nearest bin
* Clarify keyboard and mouse shortcut descriptions in the Key And
Mouse Reference window, especially for the platform-specific
descriptions used on the Mac, and add a few more alternative
shortcuts including more standard zoom in/out shortcuts
* Fix disappearing peaks in peak-bin display mode in the spectrogram,
caused by overzealous smoothing
* Fix jumps in spectrogram when changing zoom level between certain
values in hi-dpi pixel-doubled situations, caused by a failure of
cache invalidation
* Fix some very slow spectrogram painting
* Fix disappearance of right-button Transforms menu after file load
* Fix inconsistent default vs set threshold values in spectrogram
* Fix crash on painting very low-resolution dense 3d plots