add pixfader tweaks API:

* change API to hide unity/default value line
* allow to override vertical scroll [1]
* allow changing button event forwarding [2]

[1] override scrolling in track-headers
[2] dbl-click is needed for BarControllers -> switch to SpinBtn
    but a plain Slider on a track-header should not forward
    these events to the TAV (they may trigger name-edit)
gain-meter: use new PixFader API
fix 849d80b - don't show solo Lock/Iso on master bus