[AvatarButton] New component suggestion

[CumpsD]

As suggested in #836 (comment) it would be interesting to have a new component called AvatarButton which is a mix between an Avatar and a RaisedButton, so that we can have image avatars with hover over + ripple effects.

[SukantGujar]

I created a similar issue. Instead of a new component, IconButton should support Avatars.

[mbrookes]

Ref: #2913

[oliviertassinari]

We have been exposing the underlying building block of our button components: ButtonBase #7727. I can be used to build such AvatarButton on userland. I don't think that we need it in the core of the library.