[POC] Add downbeats to waveform

[alephlm]

Current UI:

Modified UI:

There are 3 new buttons:

• toggle downbeats visibility
• move downbeats forward
• move downbeats backward.

Information, Implementation details and doubts

First of all, when reviewing take in consideration that I am mainly Javascript developer. Just tired of getting the down beat wrong, then I faced c++ here. And this implementation already solve almost all my use cases.

The main concept here is the creation of a new waveformrenderdownbeat class, this is basically a copy of waveformrenderbeat but only rendering the down beats. Doing this way is less disruptive and is not messing with old code.

• It shows 4 and 8 beats differently, 8ths are generated with little triangle on top.

• There is a new skin parameter for setting the downbeat color.

• This will work only on 4/4 songs right now.

• This will only work with [x] use acceleration

• It needs a way to store the downbeatoffset after user move it.

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• I needed to get the absolute position of the beats, to do that I exposed m_beatOffset from beats.cpp. Is there already other way to do this?
• I am adding the downbeat into waveformwidget.cpp. To control visibility I am just toggling the opacity. There is maybe a quicker anyway to do? Directly removing it instead?

[acolombier]

Great to see some traction on this much needed feature!
You might want to have a look at #13330 to see points that will be expected for such a feature to be considered ready for merge