Removes JSON constructor from UI elements

This fixes the error: clang:optin.cplusplus.VirtualCall
When virtual methods are called during construction and destruction the polymorphism is restricted to the class that’s being constructed or destructed because the more derived contexts are either not yet initialized or already destructed.

UI/ui_textbox.hpp

@@ -9,7 +9,7 @@ class UI_textbox : public UI_item
     Q_OBJECT
   public:
     UI_textbox(std::string hint, Color fg_color, Color bg_color, QObject *parent = nullptr);
-    UI_textbox(const json &j, QObject *parent = nullptr);
+    UI_textbox(QObject *parent = nullptr);
 
     void set_text(std::string text) override;