Use _repr_mimebundle_ instead of _ipython_display_ message for initial display

[SylvainCorlay]

I guess this is up to debate, but using the execute_result message instead of display_data enables the display of the prompt number like for other types of outputs.

cc @Carreau @jasongrout

[SylvainCorlay]

(Test failures are expected, will fix if we decide to go this way)

[SylvainCorlay]

I wonder if there is a means to specify raw=True, or if it is even necessary for display_mime_bundle. @Carreau would probably know.

[jasongrout]

See #1811 (comment) - we can't do this just yet because of backwards compatibility.

However, see #1811 (comment) for a workaround that might work until we can assume ipython 6.1.

[jasongrout]

(fixed the description to match the code :).

[jasongrout]

(And yes, I think we should definitely move to _repr_mimebundle_ when we can)

[SylvainCorlay]

OK, I am working on the rich display mechanism for xeus-cling and I was wondering why widgets never used the execute_result message. Thanks for the pointer.

The xeus-cling rich display will only have the counterpart to _repr_mime_bundle_ from the start for all objects that have a rich representation.

[jasongrout]

The xeus-cling rich display will only have the counterpart to repr_mime_bundle from the start for all objects that have a rich representation.

Great!

[jasongrout]

However, see #1811 (comment) for a workaround that might work until we can assume ipython 6.1.

This won't work after all.

(And yes, I think we should definitely move to repr_mimebundle when we can)

Likely in ipywidgets 8.

[jasongrout]

This PR is superseded by #2021