Add a very simple list.

Do not forget to set layout of the parent component (Ext.Container
in this case). For example, here if we don't set it to 'fit',
the list won't appear.
Make our first custom component: HeaderBar

The HeaderBar is just the titlebar or toolbar
we implemented before. Here we just make it as
a custom component in a different view class
to make our code cleaner.

One thing to be noticed: require your class before
you use the xtype. Here, we have to require
`DueList.view.HeaderBar` before using `xtype: 'headerbar'`.