Fix #316754: Empty rehearsal mark not deleted after entering a line break

[mattmcclinch]

Resolves: https://musescore.org/en/node/316754.

When #4359 attempted to fix https://musescore.org/en/node/278068 (and was shortly followed up by #4364), the end result was that when a newline is inserted into a text block (causing it to split into two text blocks), the second text block would always end with a newline, even if the original text block did not. This is fixed here by setting the second text block's EOL flag to that of the original text block.

This is what is attempted by the function that joins two text blocks when a newline is deleted, but that function wasn't getting it right either. So that has been corrected here as well.

Taken together, these two errors meant that there would be a blank line at the end of any text element that had ever contained a newline. This blank line would be small, but it would always be present, even if you tried to delete it.

Because this blank was always present in multiline text elements, and because it was small enough to not attract much attention, #5881 only added an empty text fragment before a newline, and not after. But now that we do not have to have unwanted newline characters, in order to preserve the font size on a blank line, an empty text fragment may be needed after a newline also.

[Jojo-Schmitz]

This may fix it for more than just rehearasl marks, right?

(it resolves https://musescore.org/en/node/316754, not https://musescore.org/en/node/315754)

[mattmcclinch]

Yes, this fixes the issue for all text elements, not just rehearsal marks. And thanks for pointing out the typo in the link, by the way.

Of course, the thing to be aware of is that text elements in existing scores that used to have a tiny trailing blank line will now have a full-sized trailing blank line. But at least now it can be deleted, whereas up until now it could not.

[igorkorsukov]

@mattmcclinch Could you please port the changes to the master