## [IMP] mass_mailing: add new snippets

This commit adds several ready-to-use snippets to the mail editor in
`mass_mailing` to reduce the time and effort required to build emails.
These snippets provide immediate, usable building blocks and help users
understand the editor's capabilities without needing to explore or
fine-tune block options first.

To further improve discoverability and relevance, the snippet categories
are slightly reorganized and some existing snippets are revamped to better
reflect common email use cases.

Finally, this commit adds a new snippet tile in the main editor panel,
allowing users to quickly drag and drop an icon into their email.

## [IMP] *: UX Fixes and improvements to mass_mailing builder
This commit fixes some UX issues and improves upon the already in place options
and snippets.

These fixes allow for a better usage of the builder and user-friendlier options
for some of the snippets.

We reintroduced the PowerButtonsPlugin inside the mass_mailing builder which
needed adjustments for the position computations and the calls to the position
updates.
The position computations didn't take into account that the editable can be
inside an iframe. This means that the reference position between an editable in
the proper document and an iframe's document could be different as the local
overlay doesn't sit at the same position. In the case of the iframe, the local
overlay sits right beside the iframe which means that we need to not take into
account the buttons position when computing the style attribute. A compensation
for the iframeElement's DOMRect was thus added to it when in this case.

We also added the editable as permanent part of the scrollable elements inside
the `PositionPlugin`. This enables the fullscreen mode to handle scrolling to
update the position of the buttons.

This commit also removes some usages of the d-flex class as the `display: flex`
css property isn't well supported inside email client. Thus, we are starting the
cleaning process so that the convert_inline can have an easier job converting
the email to a client-friendly format.

Task-5380615

Co-authored-by: Thomas Josse (THJO) <thjo@odoo.com>
Co-authored-by: Julien Banken (JBN) <jbn@odoo.com>

Forward-Port-Of: odoo/odoo#243331