[IMP] web_editor: improve focus() for LinkDialog

Previously, LinkDialog always focused on the first editable input field.

This commit introduces a new option in the LinkDialog constructor,
`focusField`, that allows to designate a specific field to focus on.

This option is now used to focus on the second editable input field when
the first one is already filled (for example, from the user selection).

task-3224551

closes odoo/odoo#116555

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