Resolve "QLayout: Attempting to add QLayout "" to otb::Wrapper::QtWidgetParameterGroup "", which already has a layout"

Summary

Create QLayout without a parent in the constructor, as is done in QT documentation.

Rationale

Closes #1645 (closed)

Implementation Details

The QLayout constructor sets the parent's widget layout to this. In MR !125 (merged) I added this as default parent to all QObjects (including QLayouts) to avoid potential memory leaks. This is bad idea for layouts because a widget's layout cannot be changed without removing the existing one first. This causes the warning reported in #1645 (closed) for QtWidgetView and QtWidgetParameterGroup because they create a layout not intended for this. When adding a layout to this later, the warning is emitted.

Additional notes

Copyright

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

Check before merging:

• All discussions are resolved
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• The feature branch is (reasonably) up-to-date with the base branch
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