Fix filtering in various hierarchical views

The intermediate "category" nodes where never filtered out.

CategorySortFilterModel was changed to use filterRegularExpression() and
setFilterFixedString does not set that (it only affects filterRegExp()
which we need to get rid off).
Set a regular expression directly instead of a fixed string.

Amends 47e576528e

Change-Id: I6f844027b83d66ca2d5088d83e6e84b01b9eda45
Reviewed-by: hjk <hjk@qt.io>

src/plugins/mesonprojectmanager/project/buildoptions/mesonbuildsettingswidget.cpp

@@ -69,7 +69,6 @@ MesonBuildSettingsWidget::MesonBuildSettingsWidget(MesonBuildConfiguration *buil
     m_optionsFilter.setSourceModel(&m_optionsModel);
     m_optionsFilter.setSortRole(Qt::DisplayRole);
     m_optionsFilter.setFilterKeyColumn(-1);
-    m_optionsFilter.setFilterCaseSensitivity(Qt::CaseInsensitive);
 
     ui->optionsTreeView->setModel(&m_optionsFilter);
 
@@ -109,7 +108,11 @@ MesonBuildSettingsWidget::MesonBuildSettingsWidget(MesonBuildConfiguration *buil
     connect(ui->optionsFilterLineEdit,
             &QLineEdit::textChanged,
             &m_optionsFilter,
-            &QSortFilterProxyModel::setFilterFixedString);
+            [this](const QString &txt) {
+                m_optionsFilter.setFilterRegularExpression(
+                    QRegularExpression(QRegularExpression::escape(txt),
+                                       QRegularExpression::CaseInsensitiveOption));
+            });
     connect(ui->optionsTreeView,
             &Utils::TreeView::activated,
             ui->optionsTreeView,