Re-configuration support

[nazar-pc]

There are two levels to this.

First of all currently if configuration has errors, application will show blank configuration screen, while it should populate previous values instead. No notification is shown to the user about this either. This is a bit tricky due to the fact that validity of inputs is indicated in the UI and disables "Start" button, but nothing too difficult.

✔️ Second, changing configuration is currently only possible either by editing config file directly or by deleting it and re-configuring application on restart. It should be possible to change that with UI though, even if changes are only applied after restart.

✔️ Applying changes without complete restart is a bit tricky depending on what we want to do like avoiding node restart if only farmer options changed, keeping networking stack online if both node and farmer restarted (node shutdown is non-deterministic, see paritytech/substrate#11654), adding/removing farms on the fly. It should be fairly easy to change reward address though assuming we make it Arc<something>, but not clear how common that is and whether any of this is worth the implementation complexity. Probably a button for application to restart itself is sufficient here.

[nazar-pc]

Basic configuration changes are possible with #25

[nazar-pc]

With #26 it will be possible to show a restart button and restart the whole app

[nazar-pc]

With #237 we'll have ability to open configuration with old configuration of unknown validity and move from there.