Put the source combo box into a Ext.container

In the AddLayers widget, we would like to turn off the ability for
users to search for a source. This commit puts both the test and the
combo box into an ext.container which then allows use to turn it off
via CSS
Update the class name based on feedback from Bart
Merge pull request #180 from opengeo/sourceComboBox

Put the source combo box into a Ext.container