tabbox: make Thumbnail Grid active item's text bold

This makes it follow a common KDE convention of using bold text to
indicate which item in a set is active or current. Doing it here too
helps direct visual attention to the selected item, improving speed of
interaction.

src/tabbox/switchers/thumbnail_grid/contents/ui/main.qml

@@ -184,6 +184,7 @@ KWin.TabBoxSwitcher {
                         PlasmaComponents3.Label {
                             Layout.fillWidth: true
                             text: model.caption
+                            font.weight: thumbnailGridItem.isCurrentItem ? Font.Bold : Font.Normal
                             horizontalAlignment: Text.AlignHCenter
                             verticalAlignment: Text.AlignVCenter
                             textFormat: Text.PlainText