Bug 169025 Supports overridden on editor contribution
After contributting a new editor to the multiPageReportEditor, supports to enable the new editor to override a editor by assigning the ID.
UI/org.eclipse.birt.report.designer.ui.editors/src/org/eclipse/birt/report/designer/internal/ui/extension/EditorContributorManager.java
return (EditorContributor[]) editorContributorMap.values( )
				.toArray( new EditorContributor[editorContributorMap.size( )] );
	}
	public EditorContributor getEditorContributor( String targetEditorId )
	{
		Assert.isLegal( targetEditorId != null );
		synchronized ( this )
		{
			if ( editorContributorMap == null )
			{
				createEditorContributorMap( );
				if ( editorContributorMap == null )
				{
					return null;
				}
			}
		}
		return (EditorContributor) editorContributorMap.get( targetEditorId );
	}
	/**
	 * Gets the extended element point with the specified extension name.
	 * 
	 * @param extensionName
	 *            the extension name of the extended element
	 * 
	 * @return Returns the extended element point, or null if any problem exists
	 */
	public FormPageDef getFormPageDef( String targetEditorId, String pageId )
	{
		Assert.isLegal( targetEditorId != null );