Add line markers to navigate to overriding fields in delegating models

[trinhanhngoc]

In the above screenshot, product.product inherits fields of product.template by delegation mechanism (https://www.odoo.com/documentation/16.0/developer/reference/backend/orm.html#delegation). The plugin already provided line makers (the gutter icon ↑) to navigate to overridden fields: product.product::default_code => product.template::default_code, product.product::active => product.template::active,... We also need the ability to navigate in reverse direction, e.g. product.template::default_code => product.product::default_code.