Kill `monero-wallet-rpc` process on exit

[binarybaron]

We need to kill the monero-wallet-rpc process if the GUI exits. Otherwise it can become orhphaned which will prevent us from opening the wallet again later.

See this PR for more information: comit-network#1434

https://docs.rs/tauri/latest/tauri/struct.App.html#method.run

[binarybaron]

If the app exits gracefully it is killed. If we crash however it is not. I don't know if there's anything we can do about this.

[binarybaron]

Closed by #172