[TextField] - Implement helper text/error text as a separate component

[avocadowastaken]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

PR for this issue #2555, and it's also covers this #2474 issue, but I don't think that PR is complete and we need to discuss few things.

For example here I've used adjustForMultiLine and adjustForFloatingLabel "private" props that should be sent only from TextField.

Reason why I used it - it's to encapsulate component stylings in it, otherwise it would be hard to find where changed comes from - custom TextField styles, own TextField styles, custom TextFieldHelper styles or own TextFieldHelper styles.

So as I see the future implementation will look like:

<TextField helperText="Here is your tip." errorText={null} /> // Will show helper text

<TextField helperText="Here is your tip." errorText="Nay!" /> // Will show error text

or if you want to add some stylings

<TextField
    helper={<TextFieldHelper style={{color: error ? 'orange' : 'blue'}} helperText="Here is your tip." errorText={error} />}
 />

// here `TextField` should just clone element and pass `muiTheme`, `adjustForMultiLine`, `adjustForFloatingLabel` to it

Similar for all TextField child components

Some thing that can be also done here:

• Add errorStyle prop to avoid things like that - {color: error ? 'orange' : 'blue'}
• Pass ...restProps to div