Ignore slur lyrics when merging notes with Ctrl+U

[oxygen-dioxide]

Currently we can use Ctrl+U to merge notes, which will join the lyrics of all selected notes together as the lyric of the merged note. After this change, slur lyrics (lyric that starts with +) will be ignored. If all the notes are slur, it will use the lyric of the first note in selection.

For example: Original notes: {a} {+} Merged note (before this change): {a+} Merged note (after this change): {a}