[TASK]

Add hook before rendering the email template

related: #61038
[BUGFIX] Use correct RTE transformation in FlexForms

The RTE transformation configuration in the defaultExtras sections
of the FlexForm is changed from rte_transform[*] to
rte_transform[mode=ts_css].

This makes sure that the data is stored in the database in a format
that can be understodd by lib.parseFunc.

Resolves: #61118
Releases: 2.1
Change-Id: Ie58b3c97ae1b2bace4e8748f458265c3c181e5b3
[TASK] Add signal to to SendMail::createEmailBody()

A new signal slot is dispatched before the rendering of the email
template in the createEmailBody() method of the
\In2code\Powermail\Utility\SendMail class.

Additonally some PHPDoc improvements and fixes are made and an
obsolete use statement is removed.

Resolves: #61038
Releases: 2.1
Change-Id: Iba81c49ead2a287a836ace3df9c763e4d1b0c778