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
Assert.isLegal( pageId != null );
		int index = findFormPageIndex( targetEditorId, pageId );
		if ( index != -1 )
		{
			return getFormPageDef( targetEditorId, index );
		}
		return null;
	}
	public int findFormPageIndex( String targetEditorId, String pageId )
	{
		Assert.isLegal( targetEditorId != null );
		Assert.isLegal( pageId != null );
		EditorContributor editorContributor = getEditorContributor( targetEditorId );
		if ( editorContributor != null )
		{
			List formPageDefList = editorContributor.formPageList;
			if ( formPageDefList != null )
			{
				for ( int i = 0; i < formPageDefList.size( ); i++ )
				{
					FormPageDef formPageDef = (FormPageDef) formPageDefList.get( i );
					if ( formPageDef != null && formPageDef.equals( pageId ) )
					{
						return i;
					}
				}
			}
		}
		return -1;
	}