Support changing defaults for some additional props via createTheme / context

[techniq]

While it was decided to only support class/classes props as part of the initial createTheme PR, there are some additional props that could be useful to change the default globally.

• variant for many components (Button / ButtonGroup, ToggleGroup, ...)
• TextField and Field labelPlacement
• maybe others

Will need to identify which props makes sense that provides a good balance between flexibility and ease to maintain.

[techniq]

Support for some default props including labelPlacement, variant, size, etc are available per component (or base component such as Field or Button). See ComponentDefaultProps (which is used by settings() with type safety.