Piano: Add Play/Pause, Forward and Back buttons

Piano now has a toolbar allowing the playback to be paused,
or to be stepped forward or back a note.

Userland/Applications/Piano/CMakeLists.txt

@@ -6,12 +6,14 @@ serenity_component(
 )
 
 set(SOURCES
+    AudioPlayerLoop.cpp
     Track.cpp
     TrackManager.cpp
     KeysWidget.cpp
     KnobsWidget.cpp
     main.cpp
     MainWidget.cpp
+    PlayerWidget.cpp
     RollWidget.cpp
     SamplerWidget.cpp
     WaveWidget.cpp