systemtray AbstractItem: only press and hold for menu with LeftButton

It's not expected to be able to tap and hold with right mouse button

applets/systemtray/package/contents/ui/items/AbstractItem.qml

@@ -101,7 +101,7 @@ PlasmaCore.ToolTipArea {
             abstractItem.hideImmediately()
             abstractItem.pressed(mouse)
         }
-        onPressAndHold: {
+        onPressAndHold: if (mouse.button === Qt.LeftButton) {
             abstractItem.contextMenu(mouse)
         }
         onWheel: {