Locale handling of decimals in attribute saves

- Added check in controller to convert locale-specific decimal formats to use a period decimal separator.
- Added PHPdoc explanation

Signed-off-by: objecttothis <objecttothis@gmail.com>

Add TODO to clarify workaround

Signed-off-by: objecttothis <objecttothis@gmail.com>

Fixed bugs in SQL

- Added checks before attempting to delete non-existing values.
- Corrected function which deletes duplicate attribute values and replaces the attribute_ids

Signed-off-by: objecttothis <objecttothis@gmail.com>
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