It is sometimes useful in practice to be able to configure the portal so
that it looks for a target as close as possible as the portal location.
For example, in odoo, a portal may be set up in a form view that should
target the current form view. But if the form view itself is in a
dialog, it may fail, because it may find a valid target in the form view that is located
underneath.
With this commit, we add a .closest modifier to the `t-portal`
directive.
Géry Debongnie