Fixes #8046.

Fixes #7969.

The [kitty keyboard protocol][1] explicitly requires that the
*un-shifted* version of the pressed key is used to report the primary
code point in `CSI code-point;modifiers u` sequences.

> Note that the codepoint used is always the lower-case (or more
> technically, un-shifted) version of the key. If the user presses, for
> example, ctrl+shift+a the escape code would be CSI 97;modifiers u. It
> must not be CSI 65; modifiers u.

Alacritty's current behavior is to report the shifted version when shift
is pressed, and the un-shifted version otherwise:

```console
 # Note that you'll have to kill Alacritty after running this to get
 # control back!
$ echo -ne '\x1b[>1u'; cat
^[[97;5u^[[65;6u
```

The above was generated by pressing `CTRL`+`a` followed by
`CTRL`+`SHIFT`+`a` after running the command. Here `97` and `65` are the
codepoints for `a` and `A` respectively.

This change makes Alacritty match the protocol (and Kitty's) behavior.
With this change applied, `97` is reported for both `CTRL`+`a` and
`CTRL`+`SHIFT`+`a`.

[1]: https://sw.kovidgoyal.net/kitty/keyboard-protocol/#key-codes

This fixes a regression, likely introduced in 5d173f6df, which changed
the severity of missing imports from `info` back to `error`.

The cause of this issue was a more complicated error handling mechanism,
which explicitly translated IO errors to a separate enum variant without
accounting for it in all scenarios.

While retrospectively this seems completely unnecessary to me, it did
mean shorter error messages in case the main config file was not found.
To preserve the benefits of both approaches, explicit handling for the
`NotFound` IO error has been added when loading the main configuration
file.

The following config was broken:
```
[mouse]
bindings = [
    { mouse = "Right", mods = "Shift", action = "Copy"            },
    { mouse = "Right", mods = "Shift", action = "ClearSelection"  },
]
```

Only the first action was applied. Change to allow more than one exact
match in mouse mode with shift held, but keep the logic to not allow
fallback search if any exact match was found.

Regression was introduced in 1a143d11.

The user events for all cases were not handled.

Fixes: 48c088a5 (Bump winit to 0.30.0)
Fixes: #7957.

This implements a coordinated shutdown of the config monitor by sending
an event to its thread and waiting for the thread to terminate.