Fix focus and simplify popup-opener menu items

Introduce a HmenuItemPopupSpawner component that takes care of reacting
to onTriggered signals and calling the utils.makePopup() function.

The combination of this new component used in HMenu to open a HPopup
now correctly takes and restore focus to the right element when closed.

src/gui/Utils.qml

@@ -68,6 +68,11 @@ QtObject {
     }
 
 
+    function objectUpdate(current, update) {
+        return Object.assign({}, current, update)
+    }
+
+
     function objectUpdateRecursive(current, update) {
         for (const key of Object.keys(update)) {
             if ((key in current) && typeof(current[key]) === "object" &&