Rewrite dialog code to use Quick Controls 2

app/gui/NavigableDialog.qml

@@ -0,0 +1,26 @@
+import QtQuick 2.0
+import QtQuick.Controls 2.2
+
+Dialog {
+    property Item originalFocusItem
+
+    x: Math.round((window.width - width) / 2)
+    y: Math.round((window.height - height) / 2)
+
+    onAboutToShow: {
+        originalFocusItem = window.activeFocusItem
+    }
+
+    onOpened: {
+        // Force focus on the dialog to ensure keyboard navigation works
+        forceActiveFocus()
+    }
+
+    onClosed: {
+        // We must force focus back to the last item for platforms without
+        // support for more than one active window like Steam Link. If
+        // we don't, gamepad and keyboard navigation will break after a
+        // dialog appears.
+        originalFocusItem.forceActiveFocus()
+    }
+}