Improve the beat marker behaviour to allow grid stretching

[acolombier]

Currently, beats_set_marker will only add a a new grid, leading the previous grid to have an uneven number of beats. This as to address the case with varying BPM or beat/half-beat track.

Going forward, we add a new CO to allow setting a new marker, which stretches the previous grid and keep complete (that is, contain an integer of beats)

A nice to have would be to make beats_set_marker configurable to allow its behaviour to optionally copy beats_stretch's behaviour. This would allow mappings to prioritise the use of beats_set_marker, but still allow user to change the default behaviour, in case they don't use tracks with varying BPM or beat phase.

Originally discussed in #13330