Major browsers seem to preserve `white-space: pre/pre-wrap` styles in a
`<div>` when deleting the current selection through an editing command.
The idiomatic way to support this is to have a command with a "relevant
CSS property" to make sure the value is recorded and restored where
appropriate, however, no such command exists.
Create a custom command (internal to Ladybird) that implements this
behavior.
This reworks EventHandler so text insertion, backspace, delete and
return actions are now handled by the Editing API. This was the whole
point of the execCommand spec, to provide an implementation of both
editing commands and the expected editing behavior on user input.
Responsibility of firing the `input` event is moved from EventHandler to
the Editing API, which also gets rid of duplicate events whenever
dealing with `<input>` or `<textarea>` events.
The `beforeinput` event still needs to be fired by `EventHandler`
however, since that is never fired by `execCommand()`.
We do not fire `beforeinput` events since other browsers do not seem to
do so either.
The spec asks us to check whether a command's action modified the DOM
tree. This means adding or removing nodes and attributes, or changing
character data anywhere in the tree. We have
`Document::dom_tree_version()` for node updates, but for character data
a new version number is introduced that allows us to easily keep track
of any text changes in the entire tree.
The DOM spec defines what it means for an element to be an "editing
host", and the Editing spec does the same for the "editable" concept.
Replace our `Node::is_editable()` implementation with these
spec-compliant algorithms.
An editing host is an element that has the properties to make its
contents effectively editable. Editable elements are descendants of an
editing host. Concepts like the inheritable contenteditable attribute
are propagated through the editable algorithm.
The "copy" command is not in the Miscellaneous commands section. The
"defaultParagraphSeparator" command is, however. Let the accompanying
comment reflect that.
To facilitate the implementation of "delete" and all associated
algorithms, split off this piece of `Document` into a separate
directory.
This sets up the infrastructure for arbitrary commands to be supported.
