Fix Piano app (#991)

* Trying to reduce memory leaks

* Formalize the usage of pianoVK.ctrl and pianoVK.shift

* Replace all loop, make a text object list

* Add auto-garbage cleaner and remove 2 unnecessary variable

* Update app_piano.lua

parts/scenes/app_piano.lua

@@ -183,7 +183,13 @@ end
 
 function scene.update(dt)
     for _,key in pairs(pianoVK) do
-        key:update(dt)
+        key:update(nil,dt)
+    end
+
+    if lastKeyTime and keyCount>626 and TIME()-lastKeyTime>10 then
+        collectgarbage()
+        lastKeyTime=nil
+        keyCount=0
     end
 
     if lastKeyTime and keyCount>626 and TIME()-lastKeyTime>10 then