The SharedPreferences used by MidiSheetMusic.

String lastBrowsedDirectory : The last directory browsed when choosing song

When new songs are opened, we set the following MidiOptions
based on the values used in previous songs:

boolean scrollVert  : The scrolling direction (vertical/horizontal)
boolean shade1Color : The shade color
boolean shade2Color : The second shade color
boolean showPiano   : Whether the piano is shown or not

