[FIX] mass_mailing: fix unsaved changes in source mode editor

Issue

    - Install Email Marketing
    - Create/Edit a template
    - Switch into source mode
    - Add things
    - Save

    Things not saved.

Cause

    The widget override his parent `commitChanges` and don't take
    source mode into account.

Solution

    Take source mode into account like:
    https://github.com/odoo/odoo/commit/e29c08969f6e7ad981e9b7b79027d84dc89fbca2#diff-59db103a74684967379b66a89695cfdeR102-R105



OPW-2151675

closes odoo/odoo#42193

X-original-commit: 14f55b8d
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>