Support generic elements in Alto export

Refs https://gitlab.com/arkindex/requests/-/issues/733

The alto export should support all elements, not only text lines.

We'll extend the command with a new optional parameter named --layout-tag which will allow a user to select one arkindex element type. Each element found with this type should be exported using the LayoutTag attribute. The element name must be embedded as LABEL attribute, the element ID as ID attribute. (bonus, set the element name as DESCRIPTION).

If the --line-type CLI argument is also set, the potential lines of these elements must be embedded as usual (using String).