[TASK] Add mail preview function

See merge request !32

[TASK] update sys_language_uid in TCA & formatting

See merge request !31

[BUGFIX] Fix BS change of translationFile inclusions in TYPO3 10 but keep the old setting for TYPO3 9 compatibility

The userFunc was removed with dc00dbc1.
It used internal TYPO3 functions to display a simple list of the
available mail templates, so deleting the setting entirely should have no negative impact.

Feature upgrade to typo3 10

See merge request !30

[TASK] Remove exclude for l10n_parent field, because it causes bugs with translations in limited be-accounts