Bug 182714 [Contributions] Improve toolbar configurability
bundles/org.eclipse.jface/src/org/eclipse/jface/action/IContributionManagerOverrides.java
/**
     * Id for the enabled property. Value is <code>"enabled"</code>.
     * 
     * @since 2.0
     */
    public final static String P_ENABLED = "enabled"; 
    /**
     * Find out the enablement of the item
     * @param item the contribution item for which the enable override value is 
     * determined
     * @return <ul>
     * 				<li><code>Boolean.TRUE</code> if the given contribution item should be enabled</li>
     * 				<li><code>Boolean.FALSE</code> if the item should be disabled</li>
     * 				<li><code>null</code> if the item may determine its own enablement</li>
     * 			</ul>
     * @since 2.0 
     */