Refactor padding and margin handling in Form, Group, and Window constructors

Author: kojix2

src/uing/box.cr

@@ -24,7 +24,7 @@ module UIng
       else
         raise "Invalid orientation: #{orientation}"
       end
-      self.padded = true if padded
+      set_padded(true) if padded
     end
 
     def to_unsafe

src/uing/form.cr

@@ -9,7 +9,7 @@ module UIng
 
     def initialize(padded : Bool = false)
       @ref_ptr = LibUI.new_form
-      self.padded = true if padded
+      set_padded(true) if padded
     end
 
     def to_unsafe

src/uing/group.cr

@@ -9,7 +9,7 @@ module UIng
 
     def initialize(title : String, margined : Bool = false)
       @ref_ptr = LibUI.new_group(title)
-      self.margined = true if margined
+      set_margined(true) if margined
     end
 
     def to_unsafe

src/uing/window.cr

@@ -13,8 +13,9 @@ module UIng
     def initialize(@ref_ptr : Pointer(LibUI::Window))
     end
 
-    def initialize(title, width, height, has_menubar = false)
+    def initialize(title, width, height, has_menubar = false, margined : Bool = false)
       @ref_ptr = LibUI.new_window(title, width, height, has_menubar)
+      set_margined(true) if margined
     end
 
     def on_position_changed(&block : -> Void)