This is the first step to fix various MIDI edit issues for sessions with
tempo-changes.
The old code, using .earlier returned an absolute position when
calculating a relative distance. This is only valid if the session has a
fixed tempo, and the origin is irrelevant when converting the timepos.
This resulted in follow up issues since there is a difference when
summing two positions (each with an origin) vs adding an offset to a
position.
Note: this API changes breaks compilation until the GUI is updated.
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