Allow clearing notifications

[n8maninger]

Allow option to clean notifications. Currently the only way to clean notifications is to restart the application. It would be nice to be able to clear any old notifications.

From point 2 of #6

[n8maninger]

The notifications in Host Manager are designed to stay around until they are resolved either through user action or Daemon action.

The contract ones specifically will only go away once the host bugs are fixed in the Daemon. Any "problem" notifications should persist through restart.

We may switch to a more traditional notification system if that's easier for user's to manage and understand though.

Ones like the connection notification only run every 15 minutes or so. So that one would persist until the next check or a reboot even if it's been fixed. Maybe a "Check Again" button?

[n8maninger]

For now I'm marking this as "won't fix" since this would require an overhaul to how the alerting system currently works. Alerts currently are designed to require user action to resolve.

I believe Sia's built-in alerting system works basically the same way and we'll be pulling those into Host Manager's alerts as well once they're released. I think maintaining parity here is important.

Currently implemented alerts:

• Connection check failed
• Storage Low
• Collateral Low
• Not accepting contracts
• Wrong chain synced
• Wallet locked
• Wallet scanning
• Wallet balance 0
• Contract status mismatch (local node reports one status, but the blockchain reports another)

The only one in that list the host can't personally fix at the moment is "contract status mismatch". Hopefully the Sia core team will provide a way to get those cleared up in the future. 😉

I've modified the connection alert to only show up if there is a real connection issue which should stop it from being annoying.