This commit introduces the Select More/Less Operators (Ctrl +/-) for keyframes. This works like the ones for curves, by only selecting/deselecting keyframes lying in the same F-Curve. Inter F-Curve selection is not done by this operator. That is the job for another one.
This is especially useful for F-Curves set in the 0-1-0 pattern (i.e. 3 keyframes forming localised peaks), where the peaks can be selected by clicking on them individually, and immediately surrounding '0' values are selected too using "Select More".
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