[IMP] web_editor: allow options to target subelements of a snippet

Add the ability for a snippet option's method to target multiple
subelements of the option's $target using the data-apply-to attribute
with a jQuery selector. This is achieved by calling the method on a
proxy of the object where accesses to the $target are redirected to a
different object.

While it may seem preferable to just set the $target before calling the
method and restoring it after the call, as this would be much simpler
code, it is not possible as this breaks down under asynchronous
conditions, hence the need to operate on a proxy such that all
concurrent asynchronous operations have their own independent $target
available for their entire time of execution.

Part of https://github.com/odoo/odoo/pull/41402