#5 save preferences before closing project

[Roger B. Dannenberg]

prompt to save project that has changed - none of chan, solo, velocity, and mute generate a prompt to save the project

[Shuqi Dai]

commit [r527]
[Description] Save modified preferences before closing.
[Solution]
clarify the preferences we need to observe changes.
move ‘modified’ flag into Pref class.
set project_prefs directly when preferences changes.
store start_beat value, since cursor will be set in start_beat text by default, changes will always be reported once click outside the text box
[Modification]
in both serpent/prefs.srp and hcmp/midi_player/src/mp_ui.srp