Fix (and modularize) "layerification" algorithm

[yrammos]

The existing algorithm for reduction-based layer creation has a number of issues:

• it applies a music-theoretically questionable rhythmic logic;
• it is hard-wired (just like reduction algorithms, which should also be modularized);
• it respells pitches in different clefs;
• it is prone to re-bar the score.

Trivial but telling case in point:

1. Open a relatively long score without annotations.
2. Create a new layer.

In several cases, the new layer will contain respelled pitches or dislocated bar-lines.

As a short-term solution, the present algorithm could be replaced with a simpler one that:

• avoids any rhythmic "algebra" and simply replaces every reduced-out note with a rests of equal value;
• remove the "slice" option;
• remove the "tie" option.