[FEAT] Improve the overlay positions on edge

[csouchet]

• The Start position is on the first point of the edge. --> It's more readable if the position is at 5% from the start.
• The Middle position is not on the middle for the edge with several sequences and may be position on start position when changing the zoom factor. Fix in version 0.23.0, see [BUG] The 'middle' position for the overlay is not the same after zoom than after fit #1236.
• The End position is on the last point of the edge (after the arrow) --> It's more readable if the position is at 5% from the end (before the arrow).

More generally, this could be implemented by passing an offset (absolute or relative) and could also apply to the Shapes.