__References__:
Issues: https://musescore.org/en/node/19155https://musescore.org/en/node/22861https://musescore.org/en/node/23100
__Description__:
Allows to change the start and end note to which a glissando is anchored after it has been entered. Either anchor can be changed independently.
The user interface follows the current working of other 'snappable' lines. Once either the start or end grip is selected:
- `[Shift]+[Left]` snaps the anchor to the previous chord, defaulting to its top note.
- `[Shift]+[Right]` snaps to the next chord, defaulting to its top note.
- `[Shift]+[Up]` snaps to the note above (possibly in a chord, voice or staff above the current one).
- `[Shift]+[Down]` snaps to the note below (possibly in a chord, voice or staff below the current one).
This permits to set the anchor points of a glissando to any note in the score, allowing several glissandi between the notes of the same two chords and other complex configurations (glissandi skipping intermediate chords, start and end notes in different voices or staves, and so on).
It is possible to move the anchor to a different staff of the same instrument, but not to a different instrument; also, it is not possible to 'cross' a change of instrument in the same staff.
__Known limitations__:
- The `[Shift]+[Up]` and `[Shift]+[Down]` use the same note-finding functions as the `[Alt]+[Up]` and `[Alt]+[Down]`actions which move the selection cursor to the above and below note, even across voices or staves. Occasionally, in particular if the note immediately above or below is not time-aligned, the algorithm has little expected results; however, the behaviour is already known to the user. Improving the algorithm would benefit both uses.
__Notes__:
- Most of the added infrastructure is not specific to glissando but to any spanner anchored to notes, then it should also add after-the-fact "snap to" note support to note-anchored text line.
- When moving an anchor, the algorithm usually prefers a note in the same voice/staff of the old note if it exists; if there is none, it tries other voices of the same staff.
- The change of anchor is undoable.
- The fix corrects the management of the `Chord::_endsGlissando` flag, taking into account that a chord can be the ending point of several glissandi and removing one of them not necessarily means the chord no longer ends a glissando (another glissando may still exists).
- The fix also improved the rendering of the glissando wavy line, with better alignment with anchor notes and, with glissando text, better text-line spacing.
