Add option to save layered image as OpenRaster in flatten dialog

[Jaafar-Kamalah]

Related to #2285

Description of Changes

Current Progress

• Prepared the flatten dialog for a third option by returning an enum instead of a bool.
• Renamed ConfirmFlatten to ResolveFormatConflict.
• Add a third dialog response ("Change Format" or "Convert").
• Update the dialog layout so that "Convert"-option is suggested and "Flatten"-option is destructive.

Next steps

• Implement the "Save As"-flow with OpenRaster preselected for the "Convert"-option.

Checklist

• This pull request follows the project's contribution guidelines