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.

tests/UI_label.cpp

@@ -10,7 +10,8 @@ TEST(UI_label, to_from_json)
     EXPECT_EQ(j["fg_color"], "#000000FF");
     EXPECT_EQ(j["bg_color"], "#FFFFFFFF");
 
-    UI_label label2(j);
+    UI_label label2;
+    label2.from_json(j);
     EXPECT_EQ(label2.text(), "label_text");
     EXPECT_EQ(label2.fg_color(), Color(0, 0, 0));
     EXPECT_EQ(label2.bg_color(), Color(255, 255, 255));