redesign dot placement of repeat sign

redesign dot placement of repeat sign

refines6239ab9f01fcb31fbd6d5f7e81a98a7f58e663bf in that allow both
dots in the same space only as last resort (for two-line staves or for
exotic staves where all outer spaces are too narrow or asymmetric);
put no lower limit on the space required between dot and staffline.

an important example is TabStaff, which, for the present purposes, is
equivalent to a Staff with line-count 6 and staff-space 1.5.