Add confirmation of UTXOs found

[gzelda]

Move from Daemon-Technologies/Mining-Bot#100

Is your feature request related to a problem? Please describe.
One hard to diagnose issue from the chat is whether or not a miner started up with UTXOs found.
If UTXOs are not found, the miner will start as a follower, and the user still sees the chain info sync in the same fashion they would as a miner. They can only confirm the issue when the user catches up to the chain that they can observe no tBTC being spent.

Describe the solution you'd like
A small indicator that says Miner: Active or similar on the Mining Client tab, based on the output of stacks-node in the terminal window.
This way users would have a quick visual method to decide if they need to start/stop the miner again, especially since bitcoind is hard to contact at initial startup.

Describe alternatives you've considered
None.