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.
