refactor: fully remove ToolButton because no one require it.

- fully remove ToolButton.
- make loadHidpiIcon as a static function in ActionManager because no outside function may call it.

app/mainwindow.h

@@ -19,7 +19,6 @@ QT_END_NAMESPACE
 
 class ActionManager;
 class PlaylistManager;
-class ToolButton;
 class GraphicsView;
 class NavigatorView;
 class BottomButtonGroup;