ui: consider displaying balance as ? when wallet falls out of sync

[norwnd]

The idea is similar to #15 and is basically to display wallet balance as ? when we can tell that wallet fell out of sync (meaning, we prefer to show ? = unknown balance instead of stale value),

implementation notes:

• this could be especially useful during initial sync (when your balance stays at 0 until we get to the most recent blocks) and/or for users who aren't used to the details of blockchain-sync in provider-dominated world (typically provider is always synched and will display up-to-date user balance value immediately upon wallet startup)
• shouldn't switch to ? too often (only if sync is expected to take some time, maybe upon initial sync only) because that would likely look weird/jerky