Issue 4883 Implement public definition of bar-line::draw-filled-box

bar-line::draw-filled-box calls the hereby renamed procedure
bar-line::calc-blot to determine the blot diameter.
It will be used in:
  make-simple-bar-line
  make-thick-bar-line
  make-tick-bar-line
This simplifies user-customized bar-lines, which are build with
ly:round-filled-box.
No need to copy/paste the blot-calculating procedure anymore.