qml qr scanner: make xml layout look nicer

Author: SomberNight

electrum/gui/qml/android_res/layout/scanner_layout.xml

@@ -14,9 +14,16 @@
     <TextView
         android:id="@+id/hint"
         android:layout_gravity="center|top"
+        android:gravity="center"
         android:text="Scan a QR code."
         android:layout_width="wrap_content"
         android:textColor="#ffffff"
+        android:shadowColor="#000000"
+        android:shadowDx="1"
+        android:shadowDy="1"
+        android:shadowRadius="2"
+        android:textSize="15sp"
+        android:padding="14dp"
         android:layout_height="wrap_content" />
 
     <Button