Move Along Path: Option to let line position depend on start time rather than order in selection

[Myaamori]

Currently Move Along Path seems to have no regard for the start time of a line, simply incrementing the position of the lines one line at a time, in the order of the lines in the subtitle grid. This makes it harder to use Move Along Path with multi-layer signs, which would be of particular use if #8 is implemented.

It would be nice if there was an option to instead calculate the position of a line as (line_start - start) / (end - start), where start and end are the earliest and latest start times of the selected lines. (Here 0 would correspond to the start of the path, and 1 to the end of the path.) This way two lines with the same start time would end up at the same position. Combined with the "Offset existing position" option (to handle the fact that the layers of the sign may not have had exactly the same initial position), this may turn out to be a rather powerful feature.