JPEG doesn't support transparency, which was resulting in exception if a preview of an
image with transparency was made. This is now checked, and the image is converted if
necessary.
- Arguments are now grouped in a new `PageData` NamedTuple, making it simpler to use, and
easier to extend.
- Page index is automatically calculated, and can be checked in callback to know which
page to show.
- Form send by calling entity is automatically parsed, using original form to instantiate
the right field types.
- A new `DataForm.get_field` method use Generic type as second argument to retrieve the
right field when getting a `submit` form.
- Updated all classes registering Ad-Hoc commands to use the new system and simplify the
code.
