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.