[typescript] Define components as ComponentClass, not ComponentType

See e.g. Button: https://github.com/mui-org/material-ui/blob/b29f1cbec31548291a75959d3627473569343eb4/src/Button/Button.d.ts#L36.

ComponentClass supports ref= whereas ComponentType doesn’t (because the component may be a stateless functional component). So any attempt to use ref= results in a Typescript compile error.

ref’s are often used to find the anchor element for popovers and similar.

[pelotom]

For most Material UI components, the argument to a ref={...} callback would be the withStyles HOC instance… probably not what you want. It seems like a leakage of implementation details to expose that, hence why we use ComponentType everywhere, and instead expose targeted fooRef props where appropriate.

[estaub]

Ready to close, by me.