Added 'composer_full_name' and 'composer_last_name' config file values.

Set ~/.score_manager/score_manager.cfg like this:

    ...

    # Set to full name of composer.
    composer_full_name = Trevor Bača

    # Set to last name of composer.
    composer_last_name = Bača

Use: the value of composer_last_name is now used to annotate composer
stylesheets in the stylesheet library (lmy):

    Score manager - stylesheet library

        1: clean-letter-14.ily (Abjad)
        2: clean-letter-16.ily (Abjad)
        3: rhythm-letter-16.ily (Abjad)
        4: time-signature-context.ily (Abjad)
        5: baca-letter-14.ily (Bača)
        6: red-example-score-stylesheet.ily (Red Example Score)
        7: gebiete-miscellaneous.ily (Gebiete)
        8: traiettorie-header.ily (Traiettorie inargentate)
        9: traiettorie-layout.ily (Traiettorie inargentate)
        10: traiettorie-miscellaneous.ily (Traiettorie inargentate)
        11: traiettorie-paper.ily (Traiettorie inargentate)

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

    > q

The value of composer_full_name will eventually be used for automatic cover
generation.