Use PlainText for various Labels

Otherwise if a virtual desktop or window contains something that looks
like an HTML tag, it will try to render it and blow up the layout.

src/plugins/desktopgrid/qml/DesktopView.qml

@@ -295,6 +295,7 @@ FocusScope {
         rightPadding: Kirigami.Units.smallSpacing
         contentItem: PC3.Label {
             text: desktopView.desktop.name
+            textFormat: Text.PlainText
         }
         background: Rectangle {
             color: Kirigami.Theme.backgroundColor

src/plugins/overview/qml/DesktopBar.qml

@@ -198,6 +198,7 @@ Item {
                             anchors.fill: parent
                             elide: Text.ElideRight
                             text: delegate.desktop.name
+                            textFormat: Text.PlainText
                             horizontalAlignment: Text.AlignHCenter
                             verticalAlignment: Text.AlignVCenter
                             visible: label.state === "normal"

src/plugins/private/qml/WindowHeapDelegate.qml

@@ -172,6 +172,7 @@ Item {
             anchors.horizontalCenter: parent.horizontalCenter
             elide: Text.ElideRight
             text: thumb.window.caption
+            textFormat: Text.PlainText
             horizontalAlignment: Text.AlignHCenter
             verticalAlignment: Text.AlignVCenter
         }