Add expansion toggled signal of subpanels in an Accordion Panel

[DenisaCG]

Added an expansionToggled signal to the AccordionPanel class that gets triggered when an element from the AccordionPanel is collapsed or expanded.

The signal makes it useful for saving the layout of the sidebars, including the expansion status of all subpanels, such that once the user collapses a subpanel for example, the layout saves that information for later restoration.

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[DenisaCG]

Thanks for the review @fcollonval! I applied the suggested cosmetic changes and added the index of the widget for the signal to provide.

[fcollonval]

The remaining API failure is relevant and due to the extraction of the public API. To solve it, the command yarn && yarn build && yarn api must be run locally and then the changes must be committed.

I did it in 841237e

[DenisaCG]

The remaining API failure is relevant and due to the extraction of the public API. To solve it, the command yarn && yarn build && yarn api must be run locally and then the changes must be committed.

I did it in 841237e

Oh okay, that makes sense. Thank you @fcollonval!

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