BColorMenuItem: add custom color menufield

http://insightfactory.tumblr.com/image/142366356207

* Make the color box a rectangle with proportions of golden ratio.
* Override GetContentSize() to make menu item area larger.
* Label should never truncate since I make sure there is enough room.
* Draw the label using BMenuItem parent class
* Carefully adjust the spacing so that there is an attractive amount of
  padding between the checkmark and color box and the color box and label.

Add _AddMenu method to BMenuField that adds BColorMenuItem as its
base menu item. This shows the BColorMenuItem in the closed state.

Create BPrivate::MenuItemPrivate

Add a SetSubmenu() method to MenuItemPrivate that gives you
the ability to add a submenu after creating the object. This
method should be public

Skip disabled items

Color gets updated even if you select an item in a submenu

headers/os/interface/MenuField.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2015, Haiku, Inc. All rights reserved.
+ * Copyright 2006-2016 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _MENU_FIELD_H
@@ -135,6 +135,7 @@ private:
 			void				_InitMenuBar(BMenu* menu,
 									BRect frame, bool fixedSize);
 			void				_InitMenuBar(const BMessage* archive);
+			void				_AddMenu(BMenu* menu);
 
 			void				_ValidateLayoutData();
 			float				_MenuBarOffset() const;
@@ -159,4 +160,5 @@ private:
 			uint32				_reserved[2];
 };
 
+
 #endif // _MENU_FIELD_H