Fix Connections deprecation warning on Qt 5.15

Adopt the correct behavior to get rid of
"QML Connections: Implicitly defined onFoo properties in Connections
 are deprecated. Use this syntax instead:
 function onFoo(<arguments>) { ... }"

src/gui/Base/MediaPlayer/VideoPlayer.qml

@@ -42,7 +42,8 @@ Video {
 
     Connections {
         target: mainUI.fullScreenPopup
-        onClosed: fullScreen = false
+
+        function onClosed() { fullScreen = false }
     }
 
     TapHandler {