Fix spinbox locale issue (force C locale)

Summary

Force the formatting of QtWidget spinboxes to use the C locale because OTB gui applications are not i18n.

Rationale

Closes #1886 (closed)

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement