Consistent naming convention for .data() keys

[scott.gonzalez]

There's a lot of variation in naming conventions for data stored on elements. We should formalize a naming convention and use it everywhere.

[joern.zaefferer]

Started this in the data-naming branch.

[joern.zaefferer]

 https://github.com/jquery/jquery-ui/pull/607 addresses a good chunk of this.

Remaining: Widget, interactions and datepicker.

Widget currently sets data based on the widget's name. Changing that needs a BC layer.

Interactions rely a lot on other interactions, accessing them via data attributes. Haven't yet looked in detail if those are problematic.

Datepicker's usage is very similar to wiget. Looks like it just sets a datepicker property, nothing else.

[joern.zaefferer]

We'll ignore datepicker and interactions for now, as those are being rewritten. Everything else is addressed.