Add support for models that are in dB.
There's a new member `m_modelIsLinear` which can be set via the
constructor. The default value in the constructor is `true`.
Add the method `modelIsLinear` which can be used to query the parameter.
It is used in `calculateKnobPosYFromModel` and
`setVolumeByLocalPixelValue`. Both methods got extended to deal with
models in dB. They were also refactored to extract code that is common
for both cases.
Ensure that the constructor of `Fader` is called with `false` for
`CrossoverEQControlDialog` and `EqFader` because these use models that
are in dB.
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