Added (Y) command.

Description: use (Y) to edit score stylesheet with a single keystroke.

Availability: any time while working on a score.

Example:

    Score manager - example scores

        1: Blue Example Score (2013)
        2: Green Example Score (2013)
        3: Red Example Score (2013)

        scores - new (new)

    > red example

    Red Example Score (2013)

        build (u)
        distribution (d)
        makers (k)
        materials (m)
        segments (g)
        setup (p)
        stylesheets (y)

    > g

    Red Example Score (2013) - segments

        1: segment 01 (Red Example Score)
        2: segment 02 (Red Example Score)
        3: segment 03 (Red Example Score)

        all segments - current definition module - interpret (sdmi)

        all segments - current lilypond file - reinterpret (lyri)

        all segments - current pdf - version (pdfs)
        all segments - current pdf - view (pdfv)

        segments - new (new)

    > Y

    <... editor session for red_example_score/stylesheets/*stylesheet.ily ...>

Focus returns to previous location after edit.