Drop support for mutable types as dropdown options

[fterbo]

I know this has been reviewed before, but I humbly request it be reviewed again.. :-)

I don't see any upside to supporting dictionaries as an options list (you can just use widget.options = dict.items() if you want the same behaviour, which more clearly separates ownership semantics), and it is a source of many pitfalls. I suspect that many people use dictionaries simply because it's more prominently featured in the docs than the list-of-tuples approach.

I realize this is a backwards-incompatible change, but I think moving forward it would reduce community support burden (and frustrated user time). Certainly this could be done in a phased manner with DeprecationWarning (making sure users can actually see it, of course).

[jasongrout]

I definitely agree.

[jasongrout]

We can start by triggering a deprecation message when we see Mappings (e.g., dictionaries) here:

ipywidgets/ipywidgets/widgets/widget_selection.py

Lines 120 to 122 in c64f88f

	# Check if x is a mapping of labels to values
	if isinstance(x, Mapping):
	return tuple((unicode_type(k), v) for k, v in x.items())

(and change the docs and examples to not use dictionaries)

Later for 8.0 we can remove that case entirely.

[jasongrout]

Deprecation added in 7.4. We still need to actually remove support in 8.0 (i.e., a major release)