[core] Add a new classes property to all the components

[oliviertassinari]

Why

Short term goal:

• Expose the CSS as a public API. Similarly to how you can customise all the CSS of Material-UI per classes, you can now do the same per instance, unlocking maximum flexibility

Long term goal:

• Unlock the possibility to propagate theme update thanks to an event system outside of React that has his limitation
• Pave the way toward a naked Material-UI

What

• Increase the test coverage of +0.27%.
• Remove useless return in createStyleSheet. Should also dissuade destructuring the theme as it's simpler to statically analyse the usage of the theme without.
• Wrap public component around a withStyles. Should allow users to provide a classes property. And help us toward implementing a correct theme propagation rerender.
• Add a innerRef callback property to access the internal component ref
• Use a Component over a functional component for withStyles so we can get a ref
• Use hoist-non-react-statics with withStyles so we can access internal static method of Material-UI also useful with a library like next.js.
• Closes [TextField] add ref callback #6893 for the TextField and all the other components that are using a stateless functional component.
• Expose a inputRef property for the text field components so we don't have to expose focus(), blur(), select(), etc. methods.
• Closes [Stepper] Allow customization of the icon text color  #6901, [Table] Specify background colour for striped rows within a table #5286, [Dialog] Background color issue #5278, [RaisedButton] make it possible to change overlay color  #5223, [IconMenu] add a popoverStyle property #4739, 'underlineStyle' property doesn't work with 'DropDown Menu' #6698, table cannot custom thead width #6641, EnhancedButton cannot have text-transform changed #6614, Adding className to List component. #6424, Add underlineDisabledStyle to textField muiTheme object #6300, [Tab] Make active tab change background color and themable  #6271, [RaisedButton] Make hover color customizable using themes #6244, [StepLabel] custom style for states #5784, Cannot customize a Tab instance's ripple color #5576, RaisedButtton custom hover styles #5283, [Dialog] Background color issue #5278, [IconButton] Is not theme-able and the default is off spec #5190, Problem styling menuItems with selectedMenuItemStyle #5166, Style for List children #5050, Unable to style nested indicator in a list #4837, [IconMenu] add a popoverStyle property #4739, Customize icon color with themes #4696, [DropDownMenu] How to change the Popover style #4629, [InkBar] Allow override of inkBar parent div styles #3895, [ListItem] Add the ability to change the colour of the nested indicator icon #3654 as users have full access to the theme on the next branch as well as the injected style and the applied class names.
• Closes Document overrides feature from jss-theme-reactor #6687 as the overrides documentation is now fully documented.
• Closes Error when running test:regressions #6518 as it removes the baselines from the git repository and let Argos CI handle them. No more screenshot baseline update pain.
• Closes [TextField] Multiline text support #6288 as the multiline support is implemented and tested.

[oliviertassinari]

That's probably the biggest refactorization I have ever done on Material-UI. After hours of tracking down all the issues, it's green 🚀 !

[rosskevin]

Great work! Thank you for all this effort. Only two trivial flow changes from my perspective. I'm in favor of merging this ASAP.

[rosskevin]

This should probably be classes: Object, because it won't be a Function or scalar type.

[oliviertassinari]

I have tried object with no luck. thanks for the tip!

[rosskevin]

I'm curious about this style change. No objection, just curious why.

[oliviertassinari]

No real motivation, I was in that mood:

1. Using named function over anonymous one everywhere possible (we have very few, if not zero components written with arrow-function), so reducing entropy.
2. Removing intermediary variables

[rosskevin]

classes: Object

Fixed flow issue