[FIX] web_editor: adapt options' icons in RTL

The triangle pointing towards an option in the editor is pointing
right. In a RTL setting, this does not make sense and should be
mirrored.

task-3284274

closes odoo/odoo#125076

X-original-commit: f65a9bff
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>