Audit styling attributes across widgets

[jasongrout]

As brought up in discussion from #2728, it would be nice to unify some of the styling attributes various widgets have, and make sure they are used consistently across all widgets (a style background attribute was one thing brought up there, for example).

[vidartf]

Looking at the latest spec, and grepping for "StyleModel", this is their support for background field:

• ButtonStyleModel
• CheckboxStyleModel
• DescriptionStyleModel
• HTMLMathStyleModel
• HTMLStyleModel
• LabelStyleModel
• ProgressStyleModel
• SliderStyleModel
• TextStyleModel
• ToggleButtonStyleModel
• ToggleButtonsStyleModel

There are certain of these where it doesn't make sense to add the field for (e.g. button, as overriding it might make it seem unresponsive to click events), but it might be ok to add support for it, but document the potential pit fall for users?

This issue is also about potentially other fields, e.g. font_weight is just supported by 4 style models. so maybe the result of this audit should be a table of StyleModels x styling attributes.

[vidartf]

Attribute	ButtonStyleModel	CheckboxStyleModel	DescriptionStyleModel	HTMLMathStyleModel	HTMLStyleModel	LabelStyleModel	ProgressStyleModel	SliderStyleModel	TextStyleModel	ToggleButtonStyleModel	ToggleButtonsStyleModel
background		X		X	X	X			X
font_weight	X					X				X	X
button_color	X
text_decoration	X					X				X
description_width		X	X	X	X	X	X	X	X	X	X
handle_color								X
text_color	X			X	X	X			X	X
button_width											X
font_family	X					X				X
font_style	X					X				X
bar_color							X
font_variant	X					X				X
font_size	X			X	X	X			X	X

[jasongrout]

For 8.0, let's put this awesome table in the docs, and worry about filling it out more in 8.1?