#19 cannot stop player when paused

[Shuqi Dai]

commit [r527]
[Description] Player could not be stopped in 'paused' mode.
Reason: We used 'playing' variable to observe whether the player was playing or not, and process stop procedure only if the player was playing, which could not tell the situation of 'paused' (sound but not playing).
[Solution]
add 'isidle' variable in conductor/src/player.srp to observe whether player is sounding or not,
modify ‘isidle’ when player state changes in midi_player/src/mp_ui.srp and midi_player.srp