[FIX] web_editor: save on more than just input

Since commit [1], the state of the editor in the html field is only set
to dirty when the user triggers an input event. This means that if the
user uses the mouse to change the content of the field (eg, drag and
dropping snippets), the editor is not marked as dirty so the save button
will not appear and autosave will not work.

This commit fixes this by checking the editor's dirty state whenever a
history step event is triggered. Since this reverts commit [1], we need
to ensure the bug it fixed does not reappear. This is done by ensuring
we don't check the saved value against the modified editing value when
it's empty (namely, the editor doesn't allow an empty string as a value
and replaces it with `<p><br></p>`, which we need to undo in
`_isDirty`).

task-3263653
task-3288416
opw-3274329

[1]: https://github.com/odoo/odoo/pull/113088/commits/095bbb93d6c3b9b0b099b7ed1a6a15e8ad914e9f



closes odoo/odoo#120593

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>