Glissando as a multi-segment line.

- Implemented as a sub-class of `SLine`.
- Anchor type changed from CHORD to NOTE: allows to attach glissando start and end points to individual notes, rather than generically to chords (with note within the chord chosen by the program).
- The Glissando element is now stored in the `Note::_spannerFor` list.
- `Chord::_glissando` has been removed and replaced by a `bool _endsGlissando`, recording whether the chord is at the end of glissando (as gliss.-end chords require more space if mid-measure or system-initial).
- Debugger UI for `Chord` updated accordingly.
- Glissando is now saved into score file as a spanner, within the initial note, and with appropriate `&lt;endSpanner&gt;` tag in the Glissando ending note.
- Existing scores with the old Glissando file format are correctly read back.

Notes:

- MusicXML import/export of the new Glissando implementation **not implemented**.
- This version can read scores from older versions, but older versions **cannot read** scores from this version (they do not expect a &lt;Glissando&gt; tag within a Note). Does this require a **new file version number**?
- This implementation would rather easily allow to move the start and end anchors around (as for slurs) to override the note/chord chosen by the program when the glissando is initially created; but the UI for this is not implemented yet.