detecting line changes in a safer way

[eroux]

Looking at the way a line break is detected before euouae blocks, I think it's ok when there is just one score, but it leads to a completely unstable system for complex documents, where a change in the text between two scores may change the detected line break, inducing a potential change in the number of lines, itself breaking the new line detection of the following score, etc. it looks rather unsafe. I think it's necessary to detect new lines by hooking into post_linebreak_filter, just like we do for other things. This change will be too big, I think, for 4.1, but it should be a top priotity for 5.0, and this means that 5.0 will have to be out by the end of April... What do you think?

[eroux]

This is superseded by #987. There is no real problem here because once #987 will be implemented, positions of euouae blocks won't be saved anymore. The information about the difference of line or not will be saved during the first run only and won't change in other runs, so the system stays stable.