Line breaking rehearsal mark

If you need at a line break at the same time step two rehearsal marks, one rehearsal mark at the end of the line with right aligned text, and another rehearsal mark at the beginning of the next line with left aligned text, but if there is no line break these two marks should be concatenated with a space between, try this \doubleMark command. It takes two arguments (string or markup).
Additionally one of the two arguments may be the boolean ##f to indicate a begin-of-line-invisible and right alinged resp. a end-of-line-invisible and left aligned rehearsal mark, and if one of the two arguments is the boolean value ##t then the remaining text will be centered if it occurs within the line.
Override the value of gap in Score.RehearsalMark if you need a different spacing between the two text markups.
Handicaps: Also providing some functions to separately address RehearsalMarks depending on their break-direction, to tweak direction and padding.
And for compatibility when using the poly-mark-engraver (LSR No. 976) a \doublePolyMark command is provided (which otherwise will not work).