With a pruned wallet possible in master I believe we should expose the option in the GUI.

The Main tab in preferences will offer a chance to toggle the node type and size.
<img width="612" alt="pruning_gui_off" src="https://cloud.githubusercontent.com/assets/216917/8766147/834fb842-2e2c-11e5-94b0-c342132243bf.png">
<img width="612" alt="pruning_gui_on" src="https://cloud.githubusercontent.com/assets/216917/8766148/8351a0c6-2e2c-11e5-841b-af02fd77ff44.png">

The prune size UI should not permit sizes less than 550.

settings.setValue("addrSeparateProxyTor", GetDefaultProxyAddress());
    
    if (settings.value("fUseSeparateProxyTor").toBool() && !m_node.softSetArg("-onion", settings.value("addrSeparateProxyTor").toString().toStdString()))
        addOverriddenOption("-onion");
    else if(!settings.value("fUseSeparateProxyTor").toBool() && !gArgs.GetArg("-onion", "").empty())
        addOverriddenOption("-onion");
    
    if (!settings.contains("language"))
        settings.setValue("language", "");
    if (!m_node.softSetArg("-lang", settings.value("language").toString().toStdString()))
        addOverriddenOption("-lang");
    language = settings.value("language").toString();
}
/** Helper function to copy contents from one QSettings to another.
 * By using allKeys this also covers nested settings in a hierarchy.
 */
static void CopySettings(QSettings& dst, const QSettings& src)
{
    for (const QString& key : src.allKeys()) {
        dst.setValue(key, src.value(key));
    }
}
static void BackupSettings(const fs::path& filename, const QSettings& src)
{
    qWarning() << "Backing up GUI settings to" << GUIUtil::boostPathToQString(filename);
    QSettings dst(GUIUtil::boostPathToQString(filename), QSettings::IniFormat);
    dst.clear();
    CopySettings(dst, src);
}
void OptionsModel::Reset()