playlist_control: use GTimer in previousSongInPlaylist()

To determine whether to rewind the current song or to go to the
previous song, use a GTimer instead of manually diffing time(NULL).

src/playlist.c

@@ -61,12 +61,16 @@ playlist_init(struct playlist *playlist)
 
 	playlist->queued = -1;
 	playlist->current = -1;
+
+	playlist->prev_elapsed = g_timer_new();
 }
 
 void
 playlist_finish(struct playlist *playlist)
 {
 	queue_finish(&playlist->queue);
+
+	g_timer_destroy(playlist->prev_elapsed);
 }
 
 /**

src/playlist.h

@@ -82,6 +82,13 @@ struct playlist {
 	 * This variable is only valid if #playing is true.
 	 */
 	int queued;
+
+	/**
+	 * This timer tracks the time elapsed since the last "prev"
+	 * command.  If that is less than one second ago, "prev" jumps
+	 * to the previous song instead of rewinding the current song.
+	 */
+	GTimer *prev_elapsed;
 };
 
 /** the global playlist object */

src/playlist_control.c

@@ -185,15 +185,11 @@ nextSongInPlaylist(struct playlist *playlist)
 
 void previousSongInPlaylist(struct playlist *playlist)
 {
-	static time_t lastTime;
-	time_t diff = time(NULL) - lastTime;
-
-	lastTime += diff;
-
 	if (!playlist->playing)
 		return;
 
-	if (diff && getPlayerElapsedTime() > PLAYLIST_PREV_UNLESS_ELAPSED) {
+	if (g_timer_elapsed(playlist->prev_elapsed, NULL) >= 1.0 &&
+	    getPlayerElapsedTime() > PLAYLIST_PREV_UNLESS_ELAPSED) {
 		/* re-start playing the current song (just like the
 		   "prev" button on CD players) */
 
@@ -213,6 +209,8 @@ void previousSongInPlaylist(struct playlist *playlist)
 			playPlaylistOrderNumber(playlist, playlist->current);
 		}
 	}
+
+	g_timer_start(playlist->prev_elapsed);
 }
 
 enum playlist_result