applets/clipboard: set contentWidth properly

This makes it not be too wide due to the contentItem's margins, which
was an issue that went unnoticed because it was masked by always
disabling the horizontal scrollbar. Should we ever remove that override,
there won't be any regressions now.

applets/clipboard/contents/ui/Menu.qml

@@ -27,9 +27,12 @@ PlasmaComponents3.ScrollView {
     signal edit(string uuid)
     signal barcode(string text)
     signal action(string uuid)
+
     // HACK: workaround for https://bugreports.qt.io/browse/QTBUG-83890
     PlasmaComponents3.ScrollBar.horizontal.policy: PlasmaComponents3.ScrollBar.AlwaysOff
 
+    contentWidth: availableWidth - contentItem.leftMargin - contentItem.rightMargin
+
     contentItem: ListView {
         id: menuListView
         focus: true