Add option for high-res audio thumbnail

Right now the timeline's audio waveforms have a resolution of 2 points per frame, which doesn't show the peaks precisely.

An option to display it in full detail would help make more precise synchronization between sound and image.

To show you the extent of the problem, here is an image showing where some peaks of a piano track are undiscernable in Kdenlive by comparing it with Ardour:

The best would be to display it with as much precision as Ardour does. I assume it would come with a performance drag so that's why it should be an option that we can toggle only when we need frame perfect synchronization.

If this level of detail is too dangerous for large video edits, then maybe it should either be a per-clip option or simply have a resolution somewhere inbetween the current and Ardour's.

Perhaps even better would be to have 3 quality levels, one being the currently available, the second being somewhere between it and full-res and the third being full-res.

The option would probably belong in Configure Kdenlive > Timeline, as a dropdown menu.

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See #1888