XF look&feel: balance updates (either because of fiat rate change or incoming/outgoing payment) should highlight balance for 2 seconds

[knocte]

Steps to reproduce:

1. Open geewallet, see balance: it's X BTC, Y USD.
2. Put app in the background (it goes to "Sleep" mode, in Xamarin terms).
3. Some time later when BTCUSD rate has changed, put geewallet in foreground.

Current results:
Fiat balances changes. Not very easy to spot.

Expected results:
At the moment of changing the balance label text, it should be in bold for a few seconds (maybe 2) and then go back to normal, to highlight that there has been a change.