[IMP] web: add a widget property which allows to lazy load xml files
[IMP] web: add a widget property which allows to lazy load xml files

Add the 'xmlDependencies' key to the 'Widget' class. If defined, it is
an array of URLs to XML files that the widget will need to work.
Those are loaded in the `willStart` method so that they are available
before all rendering parts.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo