Feature Request: DialogTitle to accept Title and SubTitle components

[tihuan]

Context:

1. Currently <DialogTitle /> doesn't allow modifying the <Title /> and <Subtitle /> it uses in rendering the component

Solution:

1. Add TitleComponent and SubtitleComponent as new DialogTitle's props, so we can do:

const StyledDialogTitleTitle = styled(DialogTitleTitle)`
  // custom style
`

<DialogTitle title="my title" TitleComponent={StyledDialogTitleTitle} />

2. While we're there, update title and subtitle to be ReactNode to reflect the actual capability