[CardHeader] Rename actAsExpander

[alitaheri]

After the discussion in #2391, I believe the name actAsExpander is very confusing. Yet I do not know what to change it to.

Just to clarify, setting actAsExpander to true changes the CardHeader so that clicking everywhere will expand the Card. While setting it to false forces expansion only through the icon on the right.

[oliviertassinari]

The actAsExpander property was removed on the next branch. Nathan have turned that feature inside-out. We are now exposing a Collapse component to implement it. It's much better.
For instance:

<Collapse in={this.state.expanded} transitionDuration="auto" unmountOnExit>