Add support for custom combos in PrintProviderField

Hello

as for now, in plugin PrintProviderField, only the standard layout/dpi/format combos are taken into account. Any other custom combo is ignored:
https://github.com/geoext/geoext/blob/master/lib/GeoExt/plugins/PrintProviderField.js#L172-L180
whereas it is possible to add any other field (textfield for instance) thanks to:
https://github.com/geoext/geoext/blob/master/lib/GeoExt/plugins/PrintProviderField.js#L183

This PR adds the support of custom combos.
Hi Bart,
you may be right. But the ExtJS doc gives the following example that provides no "name" attribute:
```
var combo = new Ext.form.ComboBox({
    typeAhead: true,
    triggerAction: 'all',
    lazyRender:true,
    mode: 'local',
    store: new Ext.data.ArrayStore({
        id: 0,
        fields: [
            'myId',
            'displayText'
        ],
        data: [[1, 'item1'], [2, 'item2']]
    }),
    valueField: 'myId',
    displayField: 'displayText'
});
```
http://docs.sencha.com/extjs/3.4.0/#!/api/Ext.form.ComboBox

In that case, using the "valueField" param is a good fallback.