qml: automatically request focus in `MainViewLoader` loaded item

So that we don't need to use `focus: true` in every main view.
We can do this because main views are expected to request focus.

Currently views in `VideoAll` are missing this, instead of setting
them there, I thought that this approach would be less fragile.

Focus reason is a different matter and is not handled here.

modules/gui/qt/maininterface/qml/MainViewLoader.qml

@@ -224,4 +224,13 @@ Loader {
 
         sourceComponent: emptyLabel
     }
+
+    // The encapsulating Loader is a focus scope. If it has active focus,
+    // is there any reason why its loaded item would not have focus?
+    Binding {
+        target: root.currentItem
+        when: root.activeFocus
+        property: "focus"
+        value: true
+    }
 }