Selection widgets fixes

[SylvainCorlay]

Minor changes

• button_style should not be allowed to be None.
• The selected index in the selection container is allowed to be None. I think that the default value should be None (currently, it is 0, which does not make sense if no list of children was passed yet).

[jasongrout]

I think in most cases, people will expect that if they create a dropdown, the first item will be selected by default.

Perhaps we should handle the (relatively rare?) case of no children separately.

[SylvainCorlay]

The issue is that when we create a tab / accordion widget with no argument, the current default value does not make sense.

[SylvainCorlay]

I don't see a simple means to have a consistent behavior other than setting the default to None.

[SylvainCorlay]

I have rebased and removed the change to the selection container. Will open another separate PR for it for a 7.1.

[jasongrout]

Thanks!