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