SystemDialog: give minimumWidth a cap

having the width entirely dependent on the implicit size of the content
has the unfortunate side effect that a window can become insanely wide.
instead cap the minimum width at a percentage of the screen width.
shrinking beyond that cap then only allows dimension changes in height,
not width. this allows the content items to adjust accordingly (e.g.
labels would be forced to wrap).

NB: height is intentionally uncapped because we need somewhere to put
the "overflow" content

lookandfeel/contents/systemdialog/SystemDialog.qml

@@ -29,7 +29,7 @@ Item {
     implicitHeight: column.implicitHeight
     implicitWidth: column.implicitWidth
     readonly property real minimumHeight: column.Layout.minimumHeight + mainItem.implicitHeight + footerButtonBox.implicitHeight
-    readonly property real minimumWidth: column.Layout.minimumWidth + mainItem.implicitWidth + footerButtonBox.implicitWidth
+    readonly property real minimumWidth: Math.min(Math.round(Screen.width / 3), column.Layout.minimumWidth + mainItem.implicitWidth + footerButtonBox.implicitWidth)
     readonly property int flags: Qt.Dialog
     property alias standardButtons: footerButtonBox.standardButtons
     readonly property int spacing: Kirigami.Units.largeSpacing // standard KDE dialog margins