Improved rename() user experience.

Score manager now displays existing file name next to new file name for
side-by-side comparison prior to confirmation.

Example:

    Red Example Score (2013) - build directory

        1: draft.pdf
        2: draft.tex
        3: score-back-cover.pdf
        4: score-back-cover.tex
        5: score-front-cover.pdf
        6: score-front-cover.tex
        7: score-preface.pdf
        8: score-preface.tex
        9: score.pdf
        10: segment-01.ly
        11: segment-02.ly
        12: segment-03.ly

        back cover latex - edit (bce)
        back cover latex - generate (bcg)
        back cover latex - typeset (bct)
        back cover pdf - open (bco)

        draft latex - edit (de)
        draft latex - generate (dg)
        draft latex - typeset (dt)
        draft pdf - open (do)

        files - copy (cp)
        files - new (new)
        files - remove (rm)
        files - rename (ren)

        front cover latex - edit (fce)
        front cover latex - generate (fcg)
        front cover latex - typeset (fct)
        front cover pdf - open (fco)

        preface latex - edit (pfe)
        preface latex - generate (pfg)
        preface latex - typeset (pft)
        preface pdf - open (pfo)

        score latex - edit (se)
        score latex - generate (sg)
        score latex - typeset (st)
        score pdf - open (so)

        segment assembly ly - edit (sege)
        segment assembly ly - lilypond (segly)
        segment assembly pdf - open (sego)

        segment lys - copy (lycp)
        segment pdfs - copy (pdfcp)

    > ren

    Enter file to rename> 1
    Existing file name> draft.pdf
    New file name> foo.pdf

    Will rename ...

    FROM: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/draft.pdf
        TO: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/foo.pdf

    Ok?