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.cpp

@@ -5,7 +5,6 @@
 #include "mainwindow.h"
 
 #include "settings.h"
-#include "toolbutton.h"
 #include "bottombuttongroup.h"
 #include "graphicsview.h"
 #include "navigatorview.h"