Configuration properties in addition to those listed for Heron.widgets.LayerCombo.
See http://www.dev.sencha.com/deploy/dev/docs/source/TextField.html#cfg-Ext.form.TextField-emptyText, default value is “Choose a Base Layer”.
See http://www.dev.sencha.com/deploy/dev/docs/source/TextField.html#cfg-Ext.form.TextField-emptyText, default value is “Basemaps”.