tabbox/switchers/thumbnail_grid: refine icon positioning

This way it won't overlap the label when using a font with unusual
metrics.

BUG: 473690

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

@@ -154,7 +154,7 @@ KWin.TabBoxSwitcher {
                             Kirigami.Icon {
                                 anchors.horizontalCenter: parent.horizontalCenter
                                 anchors.verticalCenter: parent.bottom
-                                anchors.verticalCenterOffset: Math.round(-columnLayout.spacing/2)
+                                anchors.verticalCenterOffset: Math.round(Math.round(-thumbnailGridView.iconSize / 4))
                                 width: thumbnailGridView.iconSize
                                 height: thumbnailGridView.iconSize