IconElement.php: Rename protected "icon" property
IconElement.php: Rename protected "icon" property

This renames the "icon" property to "iconName" because the move to
traits will cause a conflict with the other "icon" property representing
the target Tag.

There will be a slight inconsistency with the JavaScript version now,
however that was already inconsistent because that used "$icon" to
represent the icon object, which we can't do in PHP.

Change-Id: I778e64592b695943eb9659665696c7a69ab27eed

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui