Sort the themes in decoration KCM

Summary:
Even though we're using a QSortFilterProxy model, by default it doesn't
actually sort anything until instructed to.

This patch turns sorting on.

Test Plan:
Opened the KCM
Looked at it

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5407

kcmkwin/kwindecoration/kcm.cpp

@@ -83,6 +83,8 @@ ConfigurationModule::ConfigurationModule(QWidget *parent, const QVariantList &ar
 {
     m_proxyModel->setSourceModel(m_model);
     m_proxyModel->setFilterCaseSensitivity(Qt::CaseInsensitive);
+    m_proxyModel->setSortCaseSensitivity(Qt::CaseInsensitive);
+    m_proxyModel->sort(0);
     connect(m_ui->filter, &QLineEdit::textChanged, m_proxyModel, &QSortFilterProxyModel::setFilterFixedString);
 
     m_quickView = new QQuickView(0);