GXM lists extend Ext.dataview.List not Ext.List.

Ext.List, which was previously used, is an alias for Ext.dataview.List. When
using the autoloading mechanism of Sencha Touch, this wouldn't correctly load
the required files for extension.
Merge pull request #56 from marcjansen/list-inheritance

GXM lists should extend 'Ext.dataview.List' not 'Ext.List'.