This allows the user to put semitransparent rectangles around the
current cursor line and column, á la Vim's cursor line/column, but
for the whole terminal.

This introduces some duplicate dependencies, though they are necessary
to build properly without any warnings.

Fixes #4735.

This refactors a large chunk of the alacritty_terminal API to expose all
data necessary for rendering uniformly through the `renderable_content`
call. This also no longer transforms the cells for rendering by a GUI
but instead just reports the content from a terminal emulation
perspective. The transformation into renderable cells is now done inside
the alacritty crate.

Since the terminal itself only ever needs to know about modified color
RGB values, the configuration for colors was moved to the alacritty UI
code.

Fixes #4634.

This adds a new visual indicator which shows the position in history of
either the display offset during search, or the vi mode cursor.

To make it as unintrusive as possible, the overlay is hidden whenever
the vi mode cursor collides with its position.

Fixes #3984.

This fixes up all of the remaining enums which are used in the
configuration file to make sure they all support fully case insensitive
deserialization.

Fixes #4611.

This replaces the existing `Deserialize` derive from serde with a
`ConfigDeserialize` derive. The goal of this new proc macro is to allow
a more error-friendly deserialization for the Alacritty configuration
file without having to manage a lot of boilerplate code inside the
configuration modules.

The first part of the derive macro is for struct deserialization. This
takes structs which have `Default` implemented and will only replace
fields which can be successfully deserialized. Otherwise the `log` crate
is used for printing errors. Since this deserialization takes the
default value from the struct instead of the value, it removes the
necessity for creating new types just to implement `Default` on them for
deserialization.

Additionally, the struct deserialization also checks for `Option` values
and makes sure that explicitly specifying `none` as text literal is
allowed for all options.

The other part of the derive macro is responsible for deserializing
enums. While only enums with Unit variants are supported, it will
automatically implement a deserializer for these enums which accepts any
form of capitalization.

Since this custom derive prevents us from using serde's attributes on
fields, some of the attributes have been reimplemented for
`ConfigDeserialize`. These include `#[config(flatten)]`,
`#[config(skip)]` and `#[config(alias = "alias)]`. The flatten attribute
is currently limited to at most one per struct.

Additionally the `#[config(deprecated = "optional message")]` attribute
allows easily defining uniform deprecation messages for fields on
structs.

This adds support for blinking the terminal cursor. This can be
controlled either using the configuration file, or using escape
sequences.

The supported control sequences for changing the blinking state are
`CSI Ps SP q` and private mode 12.