Closes #7778.

Fixes #7753.

There's no need to force alacritty's user configuration on
other users of the crate, thus provide the options actually used
by alacritty_terminal itself.

This patch replaces the mio crate with the polling. Now that
smol-rs/polling#96 has been merged, we should be at full feature parity
with mio v0.6 now.

Fixes #7104.
Fixes #6486.

Co-Authored-By: @fredizzimo
Co-Authored-By: @grueslayer

Co-authored-by: Christian Duerr <contact@christianduerr.com>

This patch adds a new mechanism for changing configuration options
without editing the configuration file, by sending options to running
instances through `alacritty msg`.

Each window will load Alacritty's configuration file by default and then
accept IPC messages for config updates using the `alacritty msg config`
subcommand. By default all windows will be updated, individual windows
can be addressed using `alacritty msg config --window-id
"$ALACRITTY_WINDOW_ID"`.

Each option will replace the config's current value and cannot be reset
until Alacritty is restarted or the option is overwritten with a new
value.

Configuration options are passed in the format `field.subfield=value`,
where `value` is interpreted as yaml.

Closes #472.

The `SizeInfo` is a SizeInfo used for rendering, which contains
information about padding, and such, however all the terminal need is
number of visible lines and columns.