Auto-adjust M-values and related events when extending beginning of line

Bud

From @AlfredBaldenweck in a related post:

One thing I'd like to keep in mind is the possibility that a line can be extended in either direction. Being able to maintain the same placement of each event when the starting point moved backwards is important to have. That is, if the starting point moved five meters backwards, I would expect the M-values to automatically account for that.