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/NavigableMenu.qml

@@ -2,13 +2,18 @@ import QtQuick 2.0
 import QtQuick.Controls 2.2
 
 Menu {
+    property var initiator
+
     onOpened: {
-        // Give focus to the first visible and enabled menu item
-        for (var i = 0; i < count; i++) {
-            var item = itemAt(i)
-            if (item.visible && item.enabled) {
-                item.forceActiveFocus(Qt.TabFocusReason)
-                break
+        // If the initiating object currently has keyboard focus,
+        // give focus to the first visible and enabled menu item
+        if (initiator.focus) {
+            for (var i = 0; i < count; i++) {
+                var item = itemAt(i)
+                if (item.visible && item.enabled) {
+                    item.forceActiveFocus(Qt.TabFocusReason)
+                    break
+                }
             }
         }
     }