Checkout: avoid displaying payment info load screen for just short moment

[danimoh]

When a currency is selected in the Multi Currency Checkout, a request is sent to the merchant server and a loading screen displayed.
As this request is often times about instant, the loading screen only quickly flashes.
To work around this, we could fade out the old view first, and then after the fade out determine whether we still have to show the loading screen or can directly fade in the new view.

[danimoh]

In the current deployment, there is also a visual glitch for that load when navigating away from the checkout page within the hub (i.e. to SignTransactionLedger).
Whether this only exists in the deployment or also in the development version has to be investigated. At least, this glitch was never observed before in the dev version.
This issue should be fixed together with the aforementioned.