Only set initial MenuItem focus if not activating via mouse

Having an initially highlighted item when using mouse navigation
doesn't adhere to UX norms and also can lead to a janky feeling
when the focus flip-flops from the item under the user's cursor
to the first item as the Menu opens.

app/gui/AppView.qml

@@ -288,6 +288,7 @@ CenteredGridView {
             asynchronous: true
             sourceComponent: NavigableMenu {
                 id: appContextMenu
+                initiator: appContextMenuLoader.parent
                 NavigableMenuItem {
                     text: model.running ? qsTr("Resume Game") : qsTr("Launch Game")
                     onTriggered: launchOrResumeSelectedApp(true)