fix(bar): improve handle monitor lifecycle handling

This commit introduces a few changes to the bar so that it can handle
the monitor disconnect/reconnect properly and so that it can map the bar
config to the correct monitor.

Previously if you had 3 monitor configs setup on `komorebi.json` with
the `display_index_preferences` set like this:

```
"display_index_preferences": [
    "0": "MONITOR_A",
    "1": "MONITOR_B",
    "2": "MONITOR_C",
]
```

But if you only had connected monitors A and C you would have to
manually change the bar configurations monitor index because now monitor
A would have index 0, but monitor C would have index 1 instead of 2.

Now with this commit this is no longer needed. Now the monitor index
setup on the bar configuration **MUST BE** the index you've used on
`display_index_preferences` for that monitor.

So in the case above you would setup the bar configurations using the
indices 0, 1 and 2 for monitors A, B and C respectively.

As for the changes introduced on this commit they are the following:

- `Komobar.monitor_index` is now an `Option`. When it is `None` it
  either means that the bar is starting and has not yet received the
  first `State` from komorebi or that the bar is disabled.

- `Komobar.config` is no longer an `Arc`. There was no need for that and
  it was creating more issues and difficulties. It was mainly used to
  pass the config as an `Arc` to `apply_config` function, but then this
  function would actually clone the config itself (not just the `Arc`).

  Also this function was passing a `self.config.clone()` most of the
  times except once when it received a new config from a hotreload. Now,
  on hotreload it first sets `self.config` to the new config and then
  calls `apply_config` which now uses its own `self.config` everywhere.

- We only change the `work_area_offset` when the bar is not disabled.

- We update the global `MONITOR` size/coordinates when we receive a
  `DisplayConnectionChange` from komorebi.

- We also check if the monitor size/coordinates have changed from the
  currently stored ones on every komorebi notification since sometimes
  the `DisplayConnectionChange` would be emitted while the state still
  had the previous size/coordinates.

  This makes sure we always capture a change of size/coordinates, store
  the new values, and update the bar.

- The previously mentioned update of the `MONITOR` coordinates also
  updates the `config.position.start` since that value is being
  overriden on `main.rs` in case the user hasn't set it so we need to
  override it again with the new monitor coordinates.

  This might mean that users of the old config system might have their
  start position changed, but if we didn't do this the bar wouldn't even
  show on the screen for them when a monitor disconnected/reconnected.

  This is another case for users to start moving into the new config
  system, since with that system the bar will still show up with the
  correct margins!