This allows an escape code to invoke user-defined extensions found in
/home/rahm/.local/bin/alacritty-ext.sh. The terminal passes to this
script the escape-code arguments.

This allows things like opening links in the browser even through an ssh
connection.

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.

This was not a _real_ violation and was _expected_, though for rust
to not complain clone FD properly...

This patch fixes an issue with signal handling where Alacritty would
permanently create one signal handling FD for each alacritty window
created by an instance. This FD was never released, causing a leak of
the FD.

Closes #7983.

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.