Add peak indicators to the mixer strips. They show the maximum peak value
that was observed and can be reset by clicking on them.
## Implementation details
The implementation works via a signal/slot mechanism. The `Fader` class
has a new signal `peakChanged` which reports peak values as
amplifications. A new class `PeakIndicator` is added which has a slot
`updatePeak` which is connected to the new signal in `Fader`.
The `PeakIndicator` inherits from `QLabel` and mainly deals with updating
the label text from the current peak value.
Add a `PeakIndicator` instance to `MixerChannelView`. Add a `reset`
method to `MixerChannelView` so that the mixer channel can be reset on
the loading of new projects, etc. The current implementation resets the
peak indicator back to -inf dbFS. The `reset` method is called in
`MixerView::clear`.
Remove the clamping in `Fader::setPeak` so that all peaks are reported.
Emit the new signal if the peak changes.
